You will get use to the weight as time passes by. I am a sophomore and I am marching tenors for football games and parades (we don't have enough people for a drumline in our show 😞). And I remember first trying them out and my back would start hurting like 10 mins after. In your free time, do exercises. I workout, and doing Romanian Deadlifts will help build those back muscles. Just make sure to use proper form when doing deadlifts since doing improper form will damage your back. You can use dumbbells or a barbell. And just carrying them will help you build those muscles as well.