The Glycogen Factor
One benefit of substituting some long runs with moderately long runs is that it enables you to increase your total training mileage without risking injury or overtraining. According to Beck, total weekly mileage is more important than the distance of the long run in marathon training. For example, a program that maxes out with a 70-mile week and a 17-mile long run is likely to render you fitter than a program that does so with a 50-mile week and a 22-mile long run.
The reason has to do with muscle glycogen, the precious fuel whose conservation is vital to marathon success. “The sheer amount of glycogen turnover inherent in high-mileage programs stimulates some of the same physiological adaptations as do long runs,” says Beck. That’s because most of the endurance-boosting benefits of a long run come toward the end, when your muscle glycogen stores are running low. But when you frequently run moderate to moderately long distances, you often start workouts with a half-empty tank.
Another way to get the benefits of 16- to 20-mile runs from 10- to 14-mile runs is to crank up the intensity of those shorter workouts. You can burn as much glycogen in a 90-minute fast run as you can in a two-plus-hour slow run, so you get a similar benefit. Beck recommends running 30 to 50 percent of the mileage in your moderately long runs at your goal marathon pace. You can also increase the intensity of these runs by running on hills instead of speeding up.