VO2-2 : VO2 Intervals -- 2x4x3(4)(15) -- 2 sets of 4 VO2 Intervals (Zone 5), each interval 3 min in duration. 4 min. rest between intervals, 15 min. rest between sets.

When you're not crushing it doing the VO2 efforts, you should be warming up or recovering in Zone 1.