Historically, runners have always spoken about and compared their training in terms of “mileage.” Counting miles alone seems a rather crude way to account for the myriad and complex effects training has on the body, especially today with the ability to track heart rate, pace, and even running power. None of that withstanding, runners still largely track their training by tallying up miles run per week.
The reason for this is simple: When it comes to endurance, volume trumps intensity. And this is especially true in long-distance running.
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Every foot strike results in a shock load to the musculoskeletal system of at least twice your body weight. That sort of pounding repeated thousands of times an hour has a big effect on the body. To the unaccustomed body, it will very quickly start to reveal some weaknesses. Overuse injuries are the most common running injuries and the most likely to derail a training program or end a running career. To the accustomed body, the positive training effect of these accumulated miles of pounding is unquestionable. There is just no other way to stimulate the training adaptations needed to handle long-distance runs.
Other aspects of your fitness, like the cardio-respiratory system and the muscle’s metabolic and strength gains will respond to training much faster which can lead to a too rapid increase in training load.
This brings us back to miles because it is the accumulation of those miles, necessarily at relatively low intensities, that causes the proper structural adaptations to occur that can allow you to run injury free for many years. Prematurely inject speedwork or other fast running before the tendons are strong and it is just a matter of time before you WILL injure yourself.
Runners and their coaches have known for well over a century that it is the miles they put on their legs that will make them faster. Famed runner and coach Alberto Salazar once explained Galen Rupp’s poor performance in one season by the fact that injury and a slight training philosophy shift had dropped his typical weeks of training into the 80-to-90-mile range from his previous 100-to-110-mile range. Sure enough, the next season when Rupp returned to his historic high-mileage base, he also returned to world-beating.
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Obviously, there was a lot more to Rupp’s successful training program than simply piling on miles. The point Salazar was making was that without the high-mileage base, Rupp could not handle the more race-specific work he needed for his track events.
In the end, despite the seeming crudity of the metric of treating miles as the Holy Grail of running, it turns out that mileage is king when it comes to preparing for a good running season.