Have a hypothesis that the usual traintraining advice doesn't work for people with relatively terrible aerobic development. Good development would be someone who's 5k pace is 7min/mile and easy pace is 9min/mile. Bad would be 7 versus 14. Or even 10 versus 16.

Supposely everyone is should do 80% of training at easy pace. One size fits all. But if you're easy is way slower than your hard the two just don't work off each other. The movement is too dissimilar. The ROM too different. Such people instead should do their "easy" at most 3 min/mile slower and make it easy by turning it into intervals. Like 11min/miles with run 4 min, walk 2. Until they eventually build up some aerobic ability.