For Z2 workouts, levels might work just fine, but with endurance work, you’re mostly looking at IF and duration.
You can’t get above .75 IF in Z2, so that’s the ceiling. Once you get there, you can only increase duration. 
For Z2 workouts, levels might work just fine, but with endurance work, you’re mostly looking at IF and duration.
You can’t get above .75 IF in Z2, so that’s the ceiling. Once you get there, you can only increase duration.