As odd as it may seem, working hard at the beginning of the workout is actually more beneficial than at the end of a workout. Working out intensely (after warm up of course) for 15 minutes and then easing off to just a “hard” level of exercise for 45 minutes will give you better results than working out hard for 45 minutes and then exercising intensely for 15. The reason for this is that most of the benefits from working out happen after you hit the “wall”. You know, that moment when you go from feeling spunky to flunky in about 10 seconds? The wall is your body’s way of telling you to ease up because you have used up you normal supply of fuel. Of course we don’t want to ease up. We want our bodies to change (either by getting less fat or more muscle or better endurance etc.) and the only way they will change is when we ask them to do what they are not capable of doing in there present state. This is what is happening after the wall. Your body is restructuring itself (and using the fat stores to do it) to better accommodate the new needs. So instead of pacing yourself early in the workout, you should instead push hard at the beginning (after warm up of course) to get the best out of your workout.
So the next time you start dragging during your class REJOICE! you hit the wall and the real work has begun.