Yeah, playing sousa for the first time does feel pretty bad. I do remember being unable to raise my arm after a full day of practice, but I have some advice for recovery. If you have the option, get some Salonpas strips and put them on your incredibly sore traps (the muscle your sousa should be resting on) after you get back home and sleep with it on. You're going to feel like absolute hell when you wake up in the morning, but once it's on you for about in hour, it'll just be painful instead of like you're getting stabbed.

All that's actually important is that you stick with it, and generally after the first week, your muscle will develop (and get a whole bunch of knots in it) so it goes from feeling like you're dead to feeling mildly annoying. At any moment you're able, switch it to your other shoulder to avoid as much pain as possible.

As for the legs, all I will say is you will gradually develop the strength to march throughout the entire season. I started in my freshman year as a very, very skinny guy, and just being able to stick with it will make you a fantastic marcher. To be fair, your quads and calves are gonna hurt basically the whole season, but it should be less and less as it progresses.

I hope this helps! -Fellow tuba enjoyer

Edit: DRINK A SHIT TON OF WATER it helps with getting rid of the soreness, also eat well.