You know how every workout DVD has a screen in the very beginning that says "check with your doctor before you begin an exercise regimen"? Well, I asked my doc about running and her advice was "I think it would be great for you". I have very bad knees, one of them has been reassembled more than once. And even still my doc's advice was "go for it". But I should also explain that my doc is a very serious athlete, she runs marathons and participates in Ironman competitions - and wins. She also happens to be 10 years older than me and in the best shape of her life.
Personally I think you have to do what is best for you. Check with your doctor, and if it's a go, start very slowly. Okay maybe long term it will be hard on your knees, but probably not any harder than carrying around all the extra weight. I suppose it's a trade off.
Starting weight: 244.6 lbs. 10/01/09
Pounds lost: 80.6 lbs
Current weight: 164 lbs
Goal weight: 120 lbs
Weight to go: 44 lbs
Goal Date: 1/1/13