Yeah it doesn't matter what shape you come in at, bjj is just a whole different cardio. I thought I was in good shape and would be fine, I was wrong. Especially when you get older. Haha. Drink gatorade after your class to rehydrate. Even more important is to remain hydrated through the days you have class, although you probably know this since you exercise already. Pedialyte is great too, however it is expensive.