There are a lot of ways to stay in shape. I am a double amputee and I too have problems wearing my prosthetics for long. The biggest thing I would change is your diet. I spend all day in a wheelchair and if I just ate whatever I wanted I would easily weigh 300 lbs. Look at other forms of exercise that you can do without putting excess stress on your stump. If it's painful to walk you either have a ill-fitting prosthetic or you need to stay off of it and let it fully heal. I handcycle,kayak, and swim to help me keep the weight off. Talk to a physical therapist who works with amputees and ask them to develop a exercise program that you can do at home. With some weights,resistance bands, medicine balls, and the big round therapy balls that you can lie or sit on to help develop your core strength. Hope this helps and let me know if you have any questions.

Do what you like and isn't a pain due to the prosthesis.

I'm no fitness guru, but I find swimming, doing the machines at the gym, and bicycling (without the prosthesis - need shoe-clip petals) are all good means of exercise and not demanding on the residual limb.