I think its important to mention that there is no magic threshold or v02 max workout. If you did the exact same Z5 workout every week, your progress will stall. Its important to alter the interval time or intensity each week. I like to do a steady build up of progressively more difficult workouts for 3 weeks, then take every 4th week easy by not riding much or doing any real workouts, but still staying active. Do hiking, play some basketball, ride bikes with your wife and kids, walk the dog for an hour, etc... That way my body can soak up all the benefits and heal itself for the next 3 week block.