Preparing for marching band

The upcoming season is going to be my first year in marching band, and I'm going to be playing the baritone (played concert euphonium for 3 years before as well). I've been practicing my baritone every day since I got it five days ago, but I've noticed that the hardest part of it is by far holding it up.

I've heard that you need to be able to hold it up for minutes on end especially during band camp, but I can barely keep it up for more than two phrases. I'm also really underweight and skinny so naturally I can't hold it up for that long.

Is there anything I can do to help me hold up my instrument for longer, or is it just a matter of building up endurance by practicing? Should I practice anything in specific?

My overall goal is to be able to hold up the baritone for longer than I currently can, so any advice would be greatly appreciated!