You might want to try out different instructors. I go to about four or five different instructors at my local gym (I go around 3-4 times a week right now) and one of them in particular does everything by "zone 1, zone 2, zone 3, and zone 4" of your heart rate. An instructor who measures exertion by heart rate might help you sustain longer and get a more satisfying workout if you're having trouble pacing yourself.