There's at least one good reason, however, for occasionally running on hard surfaces. It's probably worth going the extra mile to find forgiving surfaces to run on between more directed efforts. If you truly don't have access to soft trails, try running on the soft shoulder of a road if the surface is smooth and level. During marathon training, footfalls add up quickly, and the overall decrease in stress burden achieved by foregoing tarred roadways can be tremendous. Kellogg suggests that for these reasons, such sessions be limited to no more than one every two to three weeks for maximal effectiveness, recovery and integration into the overall training scheme.
Many runners balk at doing workouts so perilously akin to all-out racing.
Those who have the discipline can use these races for their overtempo runs. Shifts from carbohydrate to fat metabolism and the recruitment of muscle fibers in a particular sequence both occur toward the end of a prolonged work bout, especially an intense one. These conditions are difficult to mimic using other workouts. Because this increases your chances of getting to the finish line without running out of gas, it is therefore of benefit to anyone bent on peak marathon performance. One way to deplete your glycogen stores is to stop eating.