I've played and DMed for 3e, 3.5e, 4e, and Pathfinder.
I like Pathfinder and 4e both, but I'm not going to sugarcoat it for you. Going from 4e to Pathfinder can be rough, especially for a DM.
Firstly, even if you are running an AP, there is a lot of prep time necessary for the DM. First, you need to review the monsters in the encounters you anticipate the players getting to in your next session. Many of the monster abilities will be listed as feats and spells. You'll need to look these up beforehand to make sure you understand how to smoothly run them in play. Some feats are passive, but some provide active abilities. Likewise, review all the spells listed so that you are familiar with the effect and the duration. Determine whether its likely that the monster has the spell pre-cast or not. Over time, as you begin to memorize certain common spells, this prep time will be lessened, but never completely eliminated. I started out needing about an hour per encounter to review everything. Later on, I only needed about 10-15 minutes per encounter.
My players tended to be optimizers, so unless it would have been extremely unlikely, I always defaulted to the monster having the necessary buff spell already activated even if the duration was short. Also, make a note of the buffs that are active including calculating out any necessary stat changes, and be prepared to have to roll back buffs on the fly in case something gets dispelled by a player.
Some of the APs provide boosted stat blocks for the monsters, but those aren't always correct. I frequently find errors in Paizo's post-buff stat blocks so I tend to ignore them. Plus, you still need to know how the spells work anyway in case you need to adjudicate something on the fly.
Be mindful of long lasting negative effects on the PCs. Poisons, diseases, curses and the like. Paizo loves to load up players with semi-permanent debilitating status effects that can cause a later encounter to be tougher than it looks on paper if the players haven't had a chance to recover. If no one is playing a cleric, I highly recommend providing an NPC cleric of comparable level who can heal and remove such conditions. Having these effects linger for more than two sessions really frustrates and annoys players, IME.
Also, if you have a lot of players who only decide what to do when their turn comes up, and then need to flip through a ton of books, you might need to address that. It can really bog the game down, and even a core only Pathfinder game is going to be subject to it if there are casters in the group. If they need to look up spells or something, they should have done that before their turn came up. Or they should bring a laptop so they can use the online PF SRD at the table. As DM, you'll definitely want to do this as well.
Also, its not uncommon for a PC to get hit with a spell or status effect that effectively removes them from the fight, which could end up being an hour or more of real time where the player literally has nothing to do. You might set things up so they can run a monster, manage initiative or do something else to keep them engaged in the game. Otherwise, you'll get OT chatter which only exacerbates the issue in the prior paragraph of players not staying focused.
Ultimately, going back to Pathfinder broke my group in half and we never recovered. Some guys never cared for 4e and didn't want to go back, and the ones that liked 4e refused to keeping playing Pathfinder over rules frustrations, such as a failed save basically ending game night early for them.
