I'd say most of the workouts (other than one of the recovery week workouts) are anaerobic in nature. My heart rate never comes down much, even during the brief water breaks. But, there is enough of a recovery to make them do-able, even the longer ones. One thing I've changed is that I skip the long stretch (4-6 minutes) after the warmup and do it after the workout. I don't mind doing a few stretches after warming up, but the warmup ends at such a high intensity that I really don't want my heart rate to come down that much before I start the main workout.