It may not be the case for everyone, but for me there has always been a very strong correlation between my physicality and what's going on with the horn. Trumpet, at least for the gigging I do, is a very physical instrument. I've always believed that if you are doing it right, then your whole body gets into the mix - particularly the muscles of your legs and back. Lung capacity and your ability to process the air you breathe is also part of it. All of those things are improved if you are working out regularly. Something else that I believe working out helps where the horn is concerned is your stress level. The more stressed and tense you are, the more difficult it is to play - there is thread after thread, and post after post where we talk about the evils of too much tension in your playing. Working out is a great stress relief and relieves all kinds of muscle tension, so the less physically stressed you are, and the looser and more relaxed you are, the better your body is going to deal with the physical aspects of playing the horn. These are things I've picked up on over the years because I have at times been a real gym rat, and at other times I'm not working out hardly at all, and I've definitely noticed a correlation that regular exercise makes my playing easier.