ZOMBIEPOCALYPSE is not the sort of game that works well with pre-planned, structured adventures. Even in other role-playing games, the players often frustrate the GM by moving in unexpected directions, leaving the GameMaster to flail about trying to get them back on course. Fortunately, in those other games the pacing of the adventure is such that both the referee and the players have adequate time to think about their actions. This is not generally the case with ZOMBIEPOCALYPSE.
In ZOMBIEPOCALYPSE, the players are under constant pressure. Their characters are terribly vulnerable, resources are scarce and the numbers are definitely against them. It lends itself towards games where the player characters are constantly on the run, or one where the characters have found a safe place to hole up and refuse to budge. Either way, it does not lend itself to thoughtful exploration. All the effort a GM has made in a pre-planned adventure is likely to go to waste; the players will probably bypass those interesting set-pieces and will have little desire to talk to unique NPCs (especially since the latter will most likely will be killed and turned to zombies before the GM gets to reveal their most interesting traits). And attempting to railroad the players in certain directions just to ensure all the GMs work doesn't go to waste will just make the players feel restricted and controlled, which is anathema to a tabletop game whose main advantage is the freedom of choice it offers over video- or board-games.
Fortunately, in ZOMBIEPOCALYPSE extensive preparation on the part of the GM is not necessary to a successful adventure. After all, the general setting is one familiar to us all: it's Earth, albeit seasoned with a plague of the Walking Dead. It's easy to create a believable game-world; just base it on your own experiences. Think of the schools, libraries, stores, offices or homes you have visited or seen on TV; any one of them can be the location of a zombie outbreak. And because ZOMBIEPOCALYPSE games are designed to be played as separate and individual sessions, you need not worry too much about consistency; the ruined city of Pittsburgh that one group of players visits may not be the same city they revisit in another game.
In fact, almost every game of ZOMBIEPOCALYPSE that I have played has been generated "on-the-fly", with the GM inventing new locations as the players move about. When I run the session as GameMaster, I usually start it with an initial game concept - where the player characters begin and why they are together - but beyond that I have little idea as to where the game will take the players or myself. It works surprisingly well.
In order to facilitate this sort of gameplay, I've created a number of tables that other GMs can use to randomly generate the start of the adventure: the general location the player characters find themselves in, what the PCs initial relationship is to one another, what sort of zombies they will be mostly facing, that sort of stuff. With only seven die rolls, you can come up with a surprisingly deep and varied adventure hook.
Of course, these random generators are only there to set the stage; once the adventure actually gets started it's up to the GM to come up with everything else. But once you have the basic idea that the setting is a small rural town two months after the initial outbreak, most GMs can fill in the details with little difficulty.