I have pronated feet, and marched four years, back when the physical demand isn't what it is today. Even with good shoes, I ended up having a lot of problems by the end of my third year and almost couldn't march for finals. I was on a 8-ibuprofen a day habit.
My recommendation is to get orthotic insoles as soon as you can. True customs from the doctor are uber pricey. Good Feet is OK for a couple hundred. The ones hanging on the shelf with the odor eaters won't get it done.
However, I've found that the Dr. Scholl's custom fit, where you go to a Kiosk in a pharmacy and it measures your feet, work out really well for about $40. Buy two pairs, because they wear out in about 6 months - and daily marching will shorten that time.