Hello and welcome! Awesome to hear your fiance is supportive of you, its wonderful to have people at your back through the journey. It is never to late to start getting healthy, and no better motivation than your transition! I'm just returning to the site myself after having started my hormones and I want to really start working out again to get a more male shape.
I think the biggest thing to improving your health is to committing to a healthy lifestyle, rather than going on a diet. After a diet, what do you do? Go back to your old habits? No! It is good to start teaching your body what proper nutrition is

Working out at a gym can be stressful for most people, let alone for trans people. It is just as easy to get a good workout from home as it is from a gym, too. And, you have no excuses for missing a workout! You can find plenty of free youtube videos to workout to and if you don't have weights you can always checkout a thrift store or use water bottles. There are also great workout programs that you can buy that come with weights and shakes and everything you need to get started.
I have always been into health and fitness and am going to be starting a fitness challenge group to help coach and support people, keep them motivated, as well as to start taking my own advice and improve myself through my transition.
I was thinking about making a post asking more members if they would like to join so all of us in the community could support each other and create a safe space where we don't feel judged. Pre-transition, over weight, or whatever, it would be nice to have a fitness group that is pro-trans.
If that sounds like something you would like to be a part of, let me know! I would love for you to join if you think it would benefit you ^_^