You better move anyway! If you can't run, WALK! Your Dr. is correct. When you just can't move your body any more practice deep breathing exercises. Rest for a day then hit it again. I don't care how tired you are , get up and move. Even if a little bit. My experience is you will have more energy after a little bit of moving.You probably know this from experience. Just when you are getting ready to cross that finish line you get renewed energy. Use your mind to overcome the fatigue. Wash a load of clothes, do the dishes, weed the garden . . . . just move. Anything will help. You can do this especially if you are a former runner! Now you are a regular walker! It can be just as good and won't ruin your joints as you grow older. Don't give in. It will never get easier but learn to enjoy every minute. I am 68 and have dealt with this crap for the last 12 years. I still love my life and refuse to let it win. So I move.