I used to be a promising boxer when i was a kid, but got into girls and bodybuilding etc. I came out of retirement a couple of years ago after a 6 year break for 10 fights. Here's my thoughts:
Too much muscle will make you blow faster, it dosent slow you down if you can convert your strength to power, ie more speed training and plyometrics, but it will make you gas faster, as you'll have a higher oxygen debt by having more lean weight than your opponent.
Too much gear will also make you blow faster, keep the water weight off by taking non aromatising gear and keeping doses to a minimum, or just come off completely for a while. If your heart has to pump loads of water around you'll blow faster.
The other thing I noticed was the chronic adaptations of C.V training can take years to get back, people who have continiously done high intensity training from an early age can just fight non stop.
I ended up going back into retirement, but
my lesson was learnt, I'll never stop doing C.V exercise, I always get on the bag, do
some sparring and running, you never know when you'll have to run or fight or your life. Its taken me a year to get back to where I was in terms of muscle mass and weights being lifted, much faster than trying to
regain my C.V fitness.
Good luck, it's a tough sport and you need to be 100% committed, otherwise you might as well give up, in the end that's what I had to do

.