I have fibromyalgia and Reiter's Syndrome. I know a bit about pain. It hurts to move and it hurts not to move. But the more I move, the less I hurt. I have to be creative with my diet, because certain foods that work well for helping me lose weight aggravate my pain. Being poor, I also know about the "junk food is cheaper" problem in our society. Depression, stress and anxiety are factors in weight gain, so I'm working on eliminating those things, but it's a challenge to avoid stress when you are broke enough that you have to decide on whether to buy food this week, or gas for the car to get to job interviews.  
I walk. I walk three miles every day. I started out walking about a half mile, and if I miss a few days of walking, when I start up again, I can only manage about a mile and a half, but it takes less time to get back to three miles. I try to get those three miles done in an hour or less. 
Nevertheless, I gained 15 pounds since I started walking and eating better. Some fool said, "Oh, but you're gaining muscle and muscle weighs more than fat." I'm sorry. Muscle doesn't hang over your belt buckle. I had a heck of a lot more muscle when I was 30 pounds lighter than I do now. But lately, my metabolism has started increasing. I can tell. I'm hungry ALL the time. By the time I finish one meal, I'm so hungry I can't wait for the next meal. That will change, too, eventually. I don't cave in to the hunger, I just tell it to stuff itself and I go for a walk. Last week, I lost two pounds. After dealing with all the frustration, those two pounds were a godsend. I managed to get out of debt, too, so even though I'm still poor, at least I'm not in debt (well, except to my brother who loaned me the cash to pay the lawyer! LOL). Another weight off my shoulders. Amazing how much weight you carry there. I had thought most of it was in my butt. 
 
There is an advantage to having a chronic pain condition and being fairly fit. I know I'm not actually injuring myself. It's ONLY pain. It's pain for the sake of pain, not pain because some part of me is torn or bruised or otherwise damaged. I can move because the pain is there whether I move or not, so I might as well move. Since I've been in pain for over 20 years, and it's only been the last few years that it's been bad enough to slow me down, I'm much more hopeful and accepting than I was when it first hit me. I may eventually be free of the pain and not have to do battle with it anymore. Then there will be no stopping me.