Foundation increases intensity tolerance. This principle probably holds true not only for any given macrocycle, but over multiple macrocycles. The tolerance of intensity volume and frequency likely increases proportionate to the mileage base within a macrocycle, and over a period of years. The lower your base mileage, the more careful you have to be with higher intensity work. Again, Coach Conover's advise on the distribution of higher intensity work adheres to this principle by doling out the higher intensity work in terms of percent of weekly volume.
Track running poses unique stresses.
This principle is getting ahead of our subject material, as it delves into injuries as a result of environmental factors. Nevertheless, it bears mentioning at this point. Higher intensity work is often performed on the track. Generally, we run consistently in a counterclockwise fashion. Avoid excess racing.