Getting in shape for marching

I march in Mardi Gras parades on baritone for my school, and it has become apparent to me that I am NOT in shape for drum corps. The parade was about 3 1/2 hours and was about 6 miles long, and I barely survived😭 I plan on marching the 2027 season when that rolls around, and I want to be as ready as possible. Is there any warmups, stretches, workouts, etc. that could assist me in becoming more ready for the season? Any help is welcome!!