The most common mistake made by endurance athletes is to exercise so much that they can’t maintain their speed in training.
Going out and exercising slowly over long distances will not give you much endurance unless you also exercise intensely once or twice a week at much shorter distances. A person can run a marathon or ride a bicycle century much faster by training fast only twice a week. Muscle tiredness during exercise is caused primarily by damage to muscle fibers.
The best way to protect muscle fibers is to strengthen them by exercising against increasing resistance by running, cycling, or skating faster once or twice a week. But every time you exercise intensely, muscle fibers are damaged, so you have to allow time for recovery by exercising slowly the rest of the time. Pick two days for speed and the rest of the time for less intense workouts. On one day, try to work up to the point where you can do repeat very fast runs of around two minutes each. On the other day, try to exercise at a sustained fast pace for 30 to 60 minutes.
Running Research News 1990(July);6(4):4-6. 1988 (January) Grisdale; J Applied physiology 1990;69(4):1276-1282. Relative effects of glycogen depletion and previous exercise on muscle force and endurance capacity.