Let's see, as far as your knees and back are concerned, you're basically carrying around an extra person everywhere you go. Wonder why this is stressing your joints.
Can't we just, like, do our job and prescribe you something for the pain? Yes, of course opioids are the answer! I'm so glad you've independently come to the realization that not being drugged is the root of your issue. Some people would just get stuck on the whole 400 lbs thing and attempt some silly witchcraft cure like losing weight.