My DD would love to be part of a college marching band, but I wonder if it is remotely realistic for a theatre major to consider.
Most big-college marching bands (think USC, University of Michigan, etc.) grant 1 academic credit in the fall for participating in marching band, and students generally practice 3-4 days a week, for 2 or more hours at a time (i.e. 4:30 to 6:30 pm), and of course are tied up all day on Saturday home game days. They also travel to some select away games, and bowl games.
Information sites for the different bands stress that "students from every major at the college participate in the marching band" but I still wonder if it would be doable.