Are you doing just one park or both? We did both parks in one day, from 8:30 a.m.- 6 p.m. We did have Unl EP and early entry since we were staying at RPR. It was a lot of walking for sure, but we rode everything we wanted to ride in both parks at least once, and a couple of things more than once (Hagrids 3x, Mummy 2x, Hogwarts Exp (both directions), HPFJ 2x). We even stopped for an awesome full service lunch at Mythos. I think even w/o Express Pass, most rides would have been walkons or very short waits. EP really helped on Mummy, Hulk, Kong, Gringotts and Hogwarts Express. If you're just doing HP stuff, even both parks, I think you'll likely run out of things to do faster than you think if you're doing a full day. I'd check out a few other things too if I were you.