I've found 7 interval workouts that independently target endurance, muscular strength, and speed. For variety's sake, is it ok to move these around week to week or is it a rule to keep it the same all the time? For example, always do them on Tuesdays and Thursdays but swap Tempo and Lactate Threshold from T to Th every other week? Additionally, is ok to use up all 7 types over the course of a month, plugging them in here and there (obviously avoiding doing similar ones like Lactate and Muscular in the same week). Thoughts?