You're approaching exercise in a healthy way. That's big. It's amazing how many people don't listen to their doctors. Have you tried working out in the pool? It's a lot more gentle. You may find it more comfortable. Don't discount strength training either. You can modify most stretch band work to accommodate just about any issue. Many people who are in the very obese category are actually pretty strong. You have to be to carry the weight. Keep that muscle around while you lose the fat.