Mostly, yeah. 

City size? 
How big is No Man's Land? 
Is there any reason why the number of zombies [i]isn't[/i] limited? Are they reproducing or propagating in any way?
Are our characters likely to have already spent time exploring it? Perhaps they salvage for valuables?
Maybe other people try to do that as well, and that's where new zombies come from?
Will those Slayer Drones target our characters as well as the zombies? 

Are our characters immune to becoming zombies, since they've already been infected by the virus?