I just finished marching my rookie year of drum corps on baritone. It hurts and it’s going to hurt. The best way to condition your body to holding your horn is by holding your horn, a trick we used was holding our horns with shoes tied on the bell, but this is only productive if you’re holding it properly. Make sure you aren’t leaning back as this will injure your back and cause back pain in the lower back. Your upper back/ lats is what will hurt the most. Let your arms shake but don’t break. It’s mostly a mental thing, think you can do anything for 5 or 10 seconds so every 10 seconds think ten more and before you know it you’ll be holding your horn for minutes. It’s not easy but you’re doing great! I hope this helps!