I would vote swimming too. I swam on a team when I was younger and we had very tough workouts. In a pool that was cold, I would sweat and get hot but the water would wash away the sweat. I'd be sweating when I got out. Just one thing to keep in mind, the idea that swimming isn't tough on your joints isn't completely true. It actually is tough on your shoulders. I usually work out through soreness but in swimming, I was always told if your shoulders are very sore, to take it easy until they recover because you can damage them.
I would also start out swimming long distances (in different strokes if you want but you probably won't go too far in the butterfly) to build up your stamina. Once you have some endurance, then try a warm up, followed by timed sets then a cool down. I find with out doing sets, I leisurely swim for an hour and don't get as good a work out because I didn't have to push at all. You swam on a team before so you should know some sets e.g. 20 x 200 m/yd on the 2minutes 30seconds or whatever is realalistic but you only get 5 secs rest with out taking it too easy but don't sprint. As you build up from there, then you can go into some sprinting sets as well - on sprinting sets you should get more rest inbetween or do sprinting 50's alternating easy hard.
