Why the heck are you having 2 hour dates with girls you're just meeting for the first time anyway? YOU are the one setting up the date, so YOU should also be in control of how long they last. Like someone else said above, keep first dates simple - coffee shops, bookstores, someplace where you can sit and talk but also end the conversation at any time.
Anyway... the easiest way to get out of a date is to tell her BEFORE the date that you have someplace else you have to be after the date, but don't tell her what time that thing starts. That way, if the date is going well you can just say "oh, the thing got moved to tomorrow night instead;" and, if you're having a BAD time, you can say "well, I gotta get going so I can prepare for that thing I'm going to tonight." Very simple stuff, maaaaan!