I'm with Sandy State Phantom, and the other posters in this thread: More zombies (lots more! Like, 1000+ per map) but make them slower. That's the essential nature of the zombie menace in fiction: They're slow, they're stupid, but there's so many of them that this doesn't matter. Also, humans need a stamina bar. That way, while the player can run (hopefully a lot) faster than the zombies, they'll eventually get tired, but the zombies won't.