Among runners, there is typically a heavy focus on weekly mileage or running volume (e.g. hours/week*). This is with good reason, since we know that running volume is one of the strongest predictors of running performance (Yamaguchi et al., 2023). The more you run, the more muscle contractions you make, and this directly contributes to improved aerobic fitness, particularly within the muscles.

On the flip side, higher running volume is also typically (though not always) correlated with increased injury risk (Fredette et el., 2022; Viljoen et al., 2022).

Finding the right running volume for you as an individual is therefore very important to get the balance between continuing to develop fitness while avoiding injury. Many runners and coaches use the 10% rule, where weekly volume is increased by no more than 10% week-on-week.

What’s often overlooked though is how this milage should be distributed across the week in order to minimise the risk of injury, and ensure training is as effective as possible. It might be possible to improve how your volume is distributed across the week in order to increase the fitness gains you get from a given time-investment, or to reduce injury risk.

In this article, we’ll take a look at the key factors you may want to consider when planning your mileage distribution, and lay out our preferred approach.

How is running milage usually distributed? The ‘Flat Loading’ Approach.

There are, of course, many different approaches to distributing running volume. However, most intermediate and advanced runners will generally distribute their volume across at least 5-6 days per week, sometimes running multiple times in a day, with quite minimal variation in run volume each day.

This is what we refer to as a ‘flat loading’ approach.

An example is shown below for a training plan targeting 60km per week (see here for a definition of Zone 2 running):

A common feature of this flat loading distribution is that running volume is often spread relatively evenly across the days, with most runs typically lasting between ~30-60 minutes.

There may be one longer run in the week, but by-and-large most training sessions are relatively similar in terms of total distance and duration run. Since this is something these athletes tend to do week-in, week-out, these short/medium length runs are not typically distances that the athlete would consider overly challenging (though the intensity they opt to run at may make them more challenging).

If we look at the mileage or duration on a graph, it would look something like this:

Other than Monday (day off) and Sunday (long run) all other days of the week are pretty similar in terms of running volume.

This flat loading approach works well for many runners, who may have busy work and family lives, allowing them to fit in a high volume of training around other commitments.

However, while the flat loading approach might offer practicalities, we do have some concerns with this method of distributing mileage. We’ve summed these issues up below…

Problems with Flat Loading

Not enough variety in run duration

As mentioned above, overall running volume is a key factor impacting running fitness (particularly aerobic fitness). However, another key stimulus for improved running fitness come from running under conditions of lowered muscle glycogen** (Psilander et al., 2013, Olesen et al., 2010).

As glycogen levels are reduced, this triggers the expression of a gene called PGC-1α and an enzyme known as PDK4, which are responsible for a variety of aerobic adaptations, such as improved mitochondrial and capillary density and improved fat oxidation capacity.

Running at any intensity uses up muscle glycogen, though this is used more rapidly at higher intensities, or when runs are long. We generally store enough glycogen in our muscles to fuel an all-out effort lasting roughly 90-mins.

Therefore, when most runs in the week are kept to 30-60 mins as a maximum, we are rarely spending time with substantively lowered muscle glycogen levels, so we are missing out on this important training stimulus.

In contrast, if we were to polarise this running volume a little more, so that we have several runs each week that are distinctly quite long (i.e. ones that challenge our muscle glycogen stores more substantively), then the training stimulus for these runs would be more potent.

In essence, we’d get a bigger training adaptation from the same weekly mileage by making the duration of our runs more varied.

Unproductive Zone 2 runs

Zone 2 runs are low-intensity sessions performed below the ‘ventilatory threshold’, at an intensity that feels subjectively easy (no more than a 4/10 effort) and where you can hold a conversation without speaking in broken sentences. Shorter Zone 2 runs are generally sprinkled throughout a flat loading plan either to make up mileage or supposedly allow recovery between harder sessions (more on this later!).

However, if the intention of these runs is to build fitness, then these shorter Zone 2 runs are not overly productive, since they will only very minimally reduce muscle glycogen levels.

Each run you plan in a week should have a clear purpose. These short, low-intensity runs do contribute to overall volume, so they do help build running fitness to a degree (our use of the term ‘unproductive’ is a bit unfair!). However, generally we include Zone 2 runs in order to develop lower-level fitness attributes such as improved fat oxidation capacity and enhanced endurance. When most Zone 2 runs are kept to a shorter length, we don’t really hit this goal particularly well, since the training stimulus from a short Zone 2 run is pretty small, and we aren’t challenging the body’s existing endurance capabilities.

If several shorter Zone 2 runs were collated together into one longer Zone 2 run, this would allow more time to be spent training under conditions of lowered muscle glycogen and would generally challenge the athlete’s endurance abilities to a greater extent. This would make for a more potent training stimulus, and would be a far better use of the athlete’s time.

We often see the term ‘junk miles’ banded around, and while I think this term is unfair (all mileage will help build fitness to some extent), the benefits of these shorter, low-intensity runs will be quite minimal and we can in some senses see them as being ‘junk miles’ to a degree.

Not enough proper recovery

Another problem with sprinkling shorter, low-intensity runs throughout the week is that it doesn’t always allow proper recovery between key high-intensity sessions.

We often see Zone 2 runs of up to ~60-mins being described as “recovery runs”. However, running imposes a high degree of impact on the muscles and ligaments, which can be as much as 3x body weight per stride (Fredette et al., 2022). Even short runs of 30-mins or so create some fatigue. So these runs are most likely delaying rather than promoting recovery.

Building recovery into your training week is important, since this is the period of time where training adaptations actually take place. Without recovery time, you will simply impose more and more stress on the body, reducing performance, and never allowing the body to ‘bounce back’ stronger. Recovery is an essential part of training.

Recovery is also important for higher intensity training sessions. These often need to be performed at very high intensities in order to bring about the desired training adaptations. If you’re consistently going into your high-intensity sessions fatigued, you won’t be able to hit these intensities, and the session will be less productive.

In our view, recovery is best achieved by taking either a full day off, or doing some low-intensity and low-impact cross-training, such as walking, swimming or cycling, which imposes far less muscle damage than running.

Increased risk of injury

A final concern relates to the increased risk of injury that may come from running 5, 6 or even 7 days per week.

To the best of our knowledge, there aren’t any studies that have directly investigated the impact of running frequency on injury risk, while keeping weekly milage/volume constant. However, our own anecdotal experience has shown that running a little less often, and taking as much as three full days off from running per week can significantly reduce the risk of overuse injuries.

This makes physiological sense, since it allows more time between individual sessions in order for any low-level inflammation or damage to heal, and for any muscle tightness to resolve.

It could be argued that by running fewer days per week, all runs will be relatively long, which may also increase the risk of injury. However, in our personal experience, as long as the athlete builds gradually, so that they don’t jump quickly from shorter to longer runs, then this doesn’t seem to be a notable risk. This is especially true if the training is supplemented by appropriate strength and conditioning work to support the length of runs.

Our Approach: Polarised Loading

Our solution to these problems is to implement more of a polarised volume distribution.

In practice this means running at least 3x per week, but no more than 5x per week, which allows you to make each run a little longer and more challenging, while your recovery days are kept genuinely easy since they include no running at all.

We find the optimal running frequency for many athletes is 4x per week, where other activity such as walking, swimming or cycling can be done on the other days, provided the intensity is low and the sessions aren’t too long (e.g. no more than 30-90 minutes depending on fitness level and intensity).

The goal is to have at least one (ideally two) runs each week that are genuinely ‘long’. This means they are long enough to challenge your endurance capabilities (i.e. it’s a run that makes you feel tired by the end even if the intensity is low, and which requires you to eat something during). These runs will allow for time to be spent training with lowered muscle glycogen levels making more effective use of training time.

It can also be good to think of the purpose of a session when planning training. We like to classify days as either:

fitness development: a session that you find quite hard and which challenges your existing fitness level either via its duration, intensity or a combination of the two;
fitness maintenance: a session that feels easy to moderate - something you don’t have too much trouble completing and which is intended just to keep things ticking over between fitness development sessions;
recovery: a day that’s completely focussed on allowing space for the body to recover and adapt to training. You’re not trying to impose any training stress today.

Generally, we want two to four fitness development sessions per week, two to three recovery days per week, and any remaining sessions can focus on maintenance. We find this provides the optimal frequency of training stress and recovery to keep fitness progressing upwards in most athletes, irrespective of fitness level. Thinking in this way will help you to naturally polarise your weekly volume, since you should now understand that shorter runs (particularly shorter Zone 2 runs) fall into the fitness maintenance bracket so should feature fairly infrequently.

We’ve rearranged the 60km per week training plan shown above to represent a more polarised volume loading, so you can get an idea of what a polarised volume distribution might look like. The green days are recovery days, yellow are fitness maintenance and red are fitness development:

[It’s worth noting that Thursday’s session could be considered as fitness maintenance or fitness development, depending on the athlete’s existing fitness level, and there will certainly be cases where a session straddles the boundaries between the two classifications.]

If we look at a graph of the daily volume for the flat loading vs the polarised loading approaches we’ve presented for 60km/week, we can see that the polarised approach provides much more variety in training volume, and more clearly defined recovery days.

We have a few more examples below for different weekly running volumes. We’ve also shown how cross-training might be incorporated in the week:

40km/week

80km/week

100km/week

Should you do double training days?

Double training days are fairly common feature in higher-mileage training plans. Typically these are included either to enable a higher volume of running to fit around other commitments, or to keep running sessions mentally less daunting (two 45-min runs often feels easier to accomplish than a single 90-min run, for example).

Another justification for double run days is that they help reduce injury risk vs doing one longer run, since it allows a period of recovery between run segments. However, research suggests that this assumption is probably incorrect, and that injury risk may actually be heightened by performing double days (Viljoen et al. 2022).

This makes sense when we think about the timescales involved with muscle and ligament repair. Typically a double run day would have up to 11-hours of recovery between runs at most. Given muscles and ligaments take days to repair after a harder bout of running (Markus et al., 2021), then a 11-hour recovery window will do little in terms of reducing injury risk.

Moreover, by running twice per day, you’re also shortening the recovery window to the next day’s training session. For example, running in the morning and evening every day allows ~11H at most to recover, whereas running only in the mornings allows twice the amount of recovery.

It’s also very worth noting that splitting a run over two sessions is not as effective as performing that run over one single session. This comes down to the glycogen depletion argument again, since during the period between your first and second run of the day, you’re very likely to be eating(!) and topping up your muscle glycogen. This mean’s you won’t get that important stimulus from having trained with lowered muscle glycogen (or, at least, the stimulus will be less).

Sometimes though, we can’t escape the practicalities of double days. It may simply be impossible to do a long run on a particular day. If you’re implementing a polarised volume distribution with ~3 days off from running each week, then double days might be ok from an injury-risk perspective, since you are still allowing several full days each week to allow any emerging niggles to settle and repair. However, if implementing a flat loading approach, then I think double days further increase injury risk, and should be avoided if possible, since the risks will often outweigh the benefit. In this case it may be better to just run a little less mileage each week!

The bottom line is that double days are ok (but not optimal) if you’re using a polarised volume distribution, whereas they may unjustifiably increase your risk of injury and compromise training quality further if using a flat loading approach.

Footnotes

*In this post we’ll use the terms mileage, volume and duration interchangeably, though in reality volume and duration are the better terms to use since the body is only really aware of how long it’s working for, and not how much distance has been covered! Factors such as elevation, wind, ground conditions and so on can alter how long you’re running for a given mileage.

** Carbohydrates are stored in the body in the form of glycogen in the muscles and liver, as well as in the form of glucose in the blood. This glycogen is one of the main forms of fuel during running, especially running at higher intensities, but also when running for long periods of time.

References

Fredette, A., Roy, J. S., Perreault, K., Dupuis, F., Napier, C., & Esculier, J. F. (2022). The association between running injuries and training parameters: a systematic review. Journal of Athletic Training, 57(7), 650-671.

Markus, I., Constantini, K., Hoffman, J. R., Bartolomei, S., & Gepner, Y. (2021). Exercise-induced muscle damage: Mechanism, assessment and nutritional factors to accelerate recovery. European journal of applied physiology, 121, 969-992.

Olesen, J., Kiilerich, K., & Pilegaard, H. (2010). PGC-1α-mediated adaptations in skeletal muscle. Pflügers Archiv-European Journal of Physiology, 460, 153-162.

Psilander, N., Frank, P., Flockhart, M., & Sahlin, K. (2013). Exercise with low glycogen increases PGC-1α gene expression in human skeletal muscle. European journal of applied physiology, 113, 951-963.

Viljoen, Carel, Dina C. Christa Janse van Rensburg, Willem Van Mechelen, Evert Verhagen, Bruno Silva, Volker Scheer, Manuela Besomi et al. "Trail running injury risk factors: a living systematic review." British journal of sports medicine 56, no. 10 (2022): 577-587.

Yamaguchi, A., Shouji, M., Akizuki, A., Inoue, K., Fukuie, T., Sakuma, K., & Morita, I. (2023). Interactions between monthly training volume, frequency and running distance per workout on marathon time. European Journal of Applied Physiology, 123(1), 135-141.

How to plan your weekly mileage to enhance adaptations & minimise injury