Long story short, disabled vet here... Back, hip, knee, wrist and foot issues. Can't do most of the stuff I used to do in the military since my injuries and I am starting to gain a little more weight than I wanted. I've been 6 ft since I was 13 years old and about 195 through high school... Went in the military and lost 35 lbs in basic... Then I started working out more and made it to about 185 with mostly muscle. I have been out for about 6 years now and realize I am losing muscle and gaining a ton of fat and now to 215 lbs. I know chicks are digging the dad bod right now but I don't care about that.... What can a disabled person like myself do? Running and pushups are almost out of the question. Turning into a fatty... If I am on the route for much longer, I picture myself in Walmart, using an electric cart, breathing heavy with an oxygen tank keeping myself alive. I am 30 years old and should be healthy as heck but NO.... I don't want to let myself go....
As a 44 year old with a similar, but slightly different list of issues (substitute shoulder for hip and we're pretty much there) I strongly applaud you for trying to stay ahead of the game - I didn't, and it has been a struggle for me to get back to where I need to be (and I still have a ways to go). A couple of things that I would throw out there:
1. You probably already know this, but you really can't eat like you did in the military. Same thing can happen to long distance hikers (it did to me at least) - you get used to eating 6-10,000 calories a day to meet your caloric need. Once you stop the constant moving/exercise/activity, your body doesn't need that much any more, but you are still used to eating that much.
2. I am a firm believer that every person is different and you need to tailor your diet to your individual needs. I have found that for me, a diet of around 40% protein, 30% fat, and 30% carbs is the best for me to maintain my weight. It did take a good bit of trial and error to discover that, and it also took a great deal of recordkeeping. That said, with the fitness apps and calorie trackers out there, it is making it much easier.
3. As for exercise, I can't agree more with the suggestion to try swimming. No impact, helps range of motion in your joints, and you can get an incredible workout. As for muscle tone, I have had luck with fitness bands. I have been able to find exercises I can do that hit the major muscle groups but avoid the movements that cause me problems. One word of caution - take your time with any new activities and build into them slowly. I know I have a bad habit of jumping into new activities with a bit too much gusto and end up injuring myself more.
4. Take every opportunity you can that doesn't cause pain to move. With my knees/ankles, I can no longer run more than 100 yards and still be able to walk the next day, but walking isn't an issue for me (yet). So I consciously park in the farthest spot when I go out. Walking up stairs doesn't seem to hurt, but I can't walk down without pain, so whenever possible I will take the stairs up but ride the elevator down.
I know what you are going through - by the time I was 20 I had 3 knee operations, a torn muscle in my back that still plagues me, and a torn rotator cuff in my shoulder that has never healed properly. It does take a good bit more effort to stay healthy, but it is possible.