Being the big fish at your current job gives you some advantages. I would talk with your manager and explain that you need to work fewer hours so you can take better care of your physical health. Especially if you combine this with coaching your peers, they may not resent the fact that you are getting some perks they don't get. And if they do, well, that's not your problem. I have worked at many jobs where I received flexibility that others didn't have, and as long as you're not a dick about it, others will accept it because of your obvious value. The alternative is, you quit and then they get all the work.

Work isn't everything. If you start working on your health, it will improve your mood and outlook. My guess is, you have an introverted personality and occasional feelings of isolation are just part of it. Try to accept it rather than fix it, because you might find that if you did have peers, that would have its challenges too.