Two milers can run the same time off completely different engines, one built on a big base, the other on a fast finish. Anaerobic speed reserve is the number that tells them apart, and it flips the training depending on which one you've got.

VO2max gets the attention, but it's rarely what separates milers. The engine, how economically you run, and how much you've got left at the bell each pull their weight, and knowing which one is holding an athlete back is the difference between guessing and coaching.