No need to list all the benefits of exercise here but let's just mention that exercise is a great stress reliever and confidence booster. Not a bad thing for a brass player! Being in good physical shape will also improve your mental health and mood. You'll learn better and faster.
I can tell you that the older you get, the more difference it will make. In your early 20's, your mind and body are naturally young and sharp, you recover quickly and learn fast. But as you reach your 30's and the weight of the years starts adding up, it's not that easy anymore! You'll have to work for it if you want to stay fit. The good news is that exercise is fun! You might feel terrible the first few times but you'll get better and feel great after a while.
I've always been physically active on and off all my life. As I see my 40's coming soon, I decided to take it more seriously. I started to run seriously about 18 months ago on top of attending a yoga class every week. I'll do two or three runs per week plus one time yoga. At 37, I can really tell the difference in my playing. I feel stronger and more confident, I'm less tired at the end of rehearsals and concerts, my concentration is sharper and I make less little mistakes. I'm in a better mood and sleep better. Now I just can't stop running. Even in the singaporean heat!
We all like to watch sports on TV but why not getting off the couch and be your own hero sometimes?! You'll feel great and play your horn better.