I was just hired as a cashier for Winn-Dixie, but I put the wrong amount of hours on my application.?

Should I tell the hiring manager what happened as soon as possible, or should I try to work all those hours for a while until I think I can ask to have them reduced? The problem is that I'm also taking fifteen credit hours in college. I want to work between 15 and 20 hours, but I stupidly put down between 21 and 30 hours.