Your dietitian probably only equates exercise with jogging. There are alternatives. Stationary bikes for example don't require you to carry your body weight, therefore eliminating the risk of knee injuries.
As for the perks of being thin, I suggest that you focus on the perks of being
fit instead:
1. a generally stronger body
2. increased metabolic rate so you can eat properly (with the occasional junk food) without getting fat
3. increased resistance to sicknesses
4. stronger heart
5. more clothing options
6. increased self-confidence (for some people)
There are more but I think you get the idea.
Anyway, I used to be fat. When I got tired of it, I decided to undergo a crash diet for one whole year, which did the trick but made me unable to keep the weight down since I killed my metabolism in the process.
So I decided to focus on being more active. I started playing basketball regularly, swimming, working out, etc. I just kept switching to different things when I started to lose interest in one particular activity.
But now since I'm too busy to do all these things, I just go to the gym three times a week. As I've mentioned in my other posts, increased lean muscle mass = increased resting metabolic rate (i.e. you burn calories even while you're sleeping). Cardio burns fat and makes your heart stronger.
These two elements coupled with a proper diet (i.e. proper quantity, more good food, less junk) = a fitter you.
