I believe that if a run is too short to build endurance, to slow to build aerobic or anaerobic capacity, not hilly enough to build eccentric resistance, then it is toxic. These Toxic Miles do nothing but slow up the recovery from our quality runs, which reduces the intensity we can do these quality runs and make us more injury prone. Some people call these 'junk miles' as they are worthless. I prefer the stronger term, 'Toxic Miles', because I believe that they have a negative impact on an athlete's training. You can be a successful runner while putting in a lot of Toxic Miles, but they make success much harder to achieve. A measure of this is Training Monotony, which looks at the average and standard deviation of training stress, and higher values of Training Monotony have reduced benefits and increased fatigue from training. There is a place for Recovery Runs, but only when you are recovering from Delayed Onset Muscle Soreness or injury.
1 Counter Arguments
There are arguments against the 'Toxic Miles' philosophy.
- Clearing the Head Running has mental benefits beyond 'mental toughness', including a time of peace, freedom, and meditation. (See Stillness in Motion.)
- Calorie Burn The benefit of extra running for weight loss is less clear. The extra calorie burn is likely to be fairly small, but the exercise may improve the muscle's sensitivity to insulin. See Nutrient Timing. Doing cross training may be more beneficial however.
- Base Miles to Support Quality There is a belief that to perform quality training, you need to do slower miles so 'support' the speedwork. While I can see the value in building up endurance before embarking on speedwork, I have found no evidence to support the idea that doing slow running between speedwork sessions is useful.
- Base Miles to Support Long Runs There a similar belief that to perform long runs, you need slower miles as 'support'. I have seen no evidence to support this idea.
- Long Term Fatigue It is possible to do several shorter runs where the fatigue builds up to mimic the fatigue of a longer run. For instance, it's not uncommon for ultrarunners to do several ~20 mile runs with insufficient recovery instead of one longer run. This can work well, but there needs to also be a longer period of recovery to allow Supercompensation.