In the Ravenloft adventure "Feast of Goblyns" there is an encounter that includes 180 skeletons.
For some reason, that's always stuck in my mind.
In my first outline for Carrion Crown I included a note for the author of this volume to figure out a way to really capture the feel of hordes and hordes of zombies. 180 zombies, maybe?
The thing is, that with the game's tactical nature - not to mention the power level of the PCs at this point - asking the GM to throw 180 enemies on the board and roll and roll and roll every attack (especially when probably only 20s are going to actually connect) is madness.
So the zombie horde element went away... or, at least, elsewhere.
Anyway, if you want to make the second half more "zombie apocalypse," you might want to try big groups of enemies. Don't know how effective it'll be, but you know throwing a few dozen minis on the table is going to be memorable. And if the PCs blast through them, well, turns out they're badasses at this point and should get the occasional chance to feel like it.
Alternatively, although it's not a perfect system and we've never adopted a similar deal in the Pathfinder RPG, you might one to check out some of the rules for mobs (essentially "people swarms") pioneered back the Dungeon days with Shackled City and elsewhere.