How do I treat this and make myself less tense?

So I just got back from band camp today and literally, everything hurts. Back, shoulders, legs, torso, and feet. I play flute, so my arms are pretty much like wet noodles, and I've marched for at least 3 hours for 3-4 days, so my legs are like jello. The pain is really bothering me, and my back is so tense. How do I help relieve it a little?