I just went out for a ride, and after a half an hour had to pack it in. I was tired the whole way, got a stomach cramp, and just felt out of it in general. This is the first time this happens to me on the bike. I figure this kind of thing happens, as it used to when weight lifting, jogging, etc. Its just weird to get home, psyched to go out for a ride, and fall flat on your face and disappointed at the briefness of the ride.