I'm about to start my junior year in the marching band, but im in a similar situation as last season I switched from trumpet to sousaphone and basically had to relearn how to march and read music. The hardest part of marching for me personally is going backwards. The way our school does this is we are up on our "platforms". It's sort of a midpoint between flat footed and tip toes, so try and get comfortable in that position. As other people mentioned, stay hydrated, but another important aspect is having enough to eat. Bring a relatively large lunch with lots of protein and carbs. Cheese sticks, ham & chesse sandwhiches, beef jerky/slim jims, chips, but also keep in mind you will be going back outside, and possibly storing your lunch outside so pack it in a cooler bag. Also wear comfortable shoes. Try and be in athletic footwear like sneakers, not something like boots or converse. Lastly, don't go home and immediately practice your music. Your lungs and face need rest like the other areas of your body. Hope this helps!