What's marching band like?

I'm going to be a freshman in highschool next year, and I'm doing marching band. (i play clarinet) I really want to be with my friends and also do marching band stuff lol but i have some worries. I have asthma and also im not in the best shape im not fat but im not in shape ya know? (p.s. my asthma isn't really excersise induced i tend to hyperventilate is all) I'm just worried I might faint or have health problems cus its going to be really hot becus i live in california. does anyone have any advice on what i can do to get ready for marching band and prevent anything bad from happening? i walk a lot so my calve muscles are pretty strong any advice would be really appreciated and maybe you could even share your experiences in marching band 'cus people say its hard but they dont explain how its so hard thanx :)