There is no formula for optimal variation in training. As a general rule, I suggest that each week you do some running at paces ranging from a slow jog to a full sprint, at least one hilly run, and at least one off-road run (if you normally run on the roads). Exposure to Fatigue The most potent stimulus for improvements in running biomechanics is mostly likely running in a fatigued state. As we have seen, fatigue manifests as a deterioration of running form that can be measured as a reduction in entropy. When you become tired, you become unable to run the way you normally do. Your stride turns ugly. Resisting fatigue is largely a matter of trying to continue to run normally despite factors such as joint decoupling and muscle fuel scarcity that pull your stride apart. Through this effort to keep your form together, your neuromuscular system learns new patterns that increase your resistance to stride deterioration and increase your running efficiency in a rested state. The idea that running in a fatigued state is something to be sought out in training for its performance benefits is unusual. The conventional, energy-based model of running performance views the work that makes you tired as beneficial. For example, exposure to VO2max in training increases VO2max. But what I propose is that the fatigue itselfor more particularly, the effort to resist itis the point. I am not suggesting that the more fatigued running you do, the better. You could very easily run tired all the time by overtraining. There is a difference between quality fatigued running and nonproductive fatigued running. Fatigued running is quality when you have some capacity to resist it. When it goes past a certain point, fatigue wrecks your stride and there is nothing you can do about it. Any running you do in this state is unproductive. So the objective is to subject yourself to judicious doses of fatigued running in training. As a general rule, to maximize the rate of stride improvement, run in a fatigued state as much as you can without accumulating fatigue from day to day and eventually week to week.