I've been studying into this for YEARS now seeing as I was a personal trainer and well, I've come to two conclusions based off of countless little interviews with highly successful individuals and a good grasp of what the human body generally responds to(med student).
For pre-ops who are still in a mode of attempting to lose muscle, putting your body into a deliberate catabolic state by cutting meat out entirely with the exception of lean fishes and also all kinds of pasta besides rice, and any form of drink high in calories or any food high in ingredient list(thus more likely to have preservatives and other things to strain your liver in it) is the best way to go. That combined with long distance running or cycling. The best successes I've seen are from individuals who did just that, just last night i interviewed the THIRD transwoman who'd gone on a water/rice/veggies diet(plus multivitamins) and dropped from 240 pounds of muscle to 147 of skin and bones and lumps on her chest in a year, then she put italian back in her diet and her weight shot back up but in a very feminine manner, just like two others I'd interviewed who did the EXACT SAME THING.
If you're already in a feminine body but are just trying to stay healthy, scratch all the usual threats from your diet like trans fats and sodas and anything microwaved prepared, etc, just keep it natural. If you think that taking 5 hours a week(in intervals of course) to go run or cycle is a good thing to do, then do it. I know the best results I've seen as far as non-trans individuals is cycling to and from work, bringing a lot of deoderant and an extra change of clothes. My roommate is currently doing that and he's simply doing amazingly.
If you're still not seeing results then keep up your cardio but vary it in intervals. Sprinting or uphill cycling for example, switching things up to do whatever makes you breathe harder, even if you can only do it for so long.
If you're already postop, I don't see any issues with doing some pushups and minor, high rep resistant work on the lower body in the gym. I'd just be careful because if you're still looking to lose muscle, this won't help you, but if you've already lost it, then go for it.
Sorry I couldn't be of more help. I'm still working on an official essay once I can include my own success story in it on MtF dieting and living a healthy lifestyle, as always anyone and everyone is free to PM me concerning advice they need and/or successes so that I could learn a bit more to contribute back into the community.