Good luck with this. In my experience, it is hard to exercise just for the sake of exercising, and there aren't that many ways to disguise it as a fun activity. So far, I have found one thing that I will do for exercise, and that is playing Dance Dance Revolution. However, since you said that the marching would probably kill you now, DDR is probably not for you.
Another thing that "worked" for me was drastically reducing caloric intake, while exercising way more than was healthy. I think at one point I was biking a dozen or so miles a day, while only consuming around 500 calories. Not a healthy way to lose weight at all, but I dropped the pounds. Probably nearly killed myself, too. Not something I would recommend other people try at all.

Anyway, again I say Good Luck, and don't be discouraged by the difficulty of the task at hand.