Marine Band info
I am currently serving in the Marine Band at Quantico. I have been in for 6 years and hate seeing people be misinformed. To be in the Marine band, you first audition, if you don't make it, then you don't have to enlist or you can change jobs or have another audition later. There are 12 field bands each with it's own mission. Five of the twelve have deployed and will probably deploy in the future. As far as being in "top physical condition," I'm sure each branch will bring you up to their standards, I know the Marine do for sure. Once accepted, you go through boot camp, combat training, then the School of Music, all and all it takes about a year before your first band.
No matter where you decide to go, don't enlist for just playing, because you will be tasked to do other things besides play. I have seen many good musicians do poorly because they were jaded because they couldn't just play.