I work out a lot, and i participate in a TRX class that is an hour long with pretty much no breaks. I have seen 5-6 people join the class then quit after the first one, not because it was too hard (weight wise, TRX is a bodyweight suspension trainer), but because their lungs couldnt keep up with the intensity of the class. Like what was said above, if you are a couch potato dont expect to start doing insanity/p90x right off the bat or you will kill yourself. I worked out for quit a while, but never saw the results i got when i changed my diet to <1400 calories for 2.5 months. Went from 195 20% bodyfat to 178lbs 8% bodyfat now and gaining (muscle that is).