The following refers to weight training and developing larger muscles specifically, however it is a good way of explaining why runners should completely vary their training from day to day, week to week, month to month in order to improve.
Hypertrophy Strength And Hypertrophy:
The Important Difference
Hypertrophy strength - also called functional hypertrophy - is the increased ability to exert force as a direct result of greater functional mass, or muscle tissue. The term hypertrophy refers to the increase in size of muscle cells, as well as to the increase in the number of contractile (muscle) fibers.
It is important to understand that hypertrophy occurs, to some degree, at all levels of resistance training, but, the amount of hypertrophy an individual experiences at any intensity bracket is a function of their genetic predispositionand fiber type.
Therefore, the greatest amounts of hypertrophy do not occur in the same intensity bracket for everyone alike. This factor, and the ability to manipulate it to athletic advantage, is the most important concept in strength coaching today.
Hypertrophy relies on the principle of progressive overload, which states that the body adapts and becomes resistant to the same training load over time. In order to make progress, one needs to constantly change the, distances, reps, sets and rest period. This can be interpreted for runners as varying training each week by running at different distances, different speed, different terrains, different gradients and so on. A runner who trains the same distances, same speeds and same gradients every week is NOT training, they are just running.