I would also reccomend tai chi and wing chun, but you should remember that although in the long term these may help your health, they wont provide the strenuous excersise your doctor is reccomending. Personally, I would reccomend joining a gym, where you can choose what level of excersise you want, or get a personal trainer. Also there will be dedicated medical staff around to help you incase of a problem, which is not gaurranteed (did i spell that right?) at a martial arts class.