The way the interviewing process works is that they take a field of candidates who can do the job, and whittle it down a few times. Getting a callback means you're totally qualified, which means you have *nothing* to fear from this point on. Be confident -- dress and act like you're applying to be a manager. From this point on, they're curious about how well you'll fit in and how much potential you've got. Obviously you want to give the impression that you've got lots of potential (because you probably do). 

TLDR -- you did good and you should feel good. Odds are on your side now.