If you have the material, you'll have little trouble. It runs like 3.x for the most part. There are numerous mechanical differences, and some of them make little sense. It's a little like running 3.5 the first time after being really familiar with 3.0. There are enough things that are different that you'll probably stumble into them, but if you run them in the 'wrong' way, you won't break the game.
Pathfinder attempted to 'balance' the base classes of 3.x with later 'splat classes' and claimed 'backward compatability' as a goal. In general, I'd recommend running just 'core Pathfinder' and not allowing 3.x splats - while they might work, they need some careful review, and that's really only possible if you're familiar with the system.
Just as in 3.x, casters remain supreme. They have new ways to be even more awesome. Forget about trying to 'reign in' the casters - you're going to have make challenges based on their abilities and try to help everyone else keep up if it looks like they're falling behind.