[quote]there would be way more than just 10 on us[/quote]

Debatable, in my experience with zombie fiction and survival horror long sustained groups larger than [i]six people[/i] aren't likely outside of established survival communities (Wellington in [i]The Walking Dead[/i], Boston in [i]The Last of Us[/i], Argus Research Campus in [i]Zombie Hunters[/i], The Enclave in [i]State of Decay[/i], and so on).

[quote] so cars, lots of guns,[/quote]

Fuel would be stripped by lots of survivors, some gas would be lost through other means and needed for other things outside of cars (electric generators for example). Ammo would also be difficult-ish to obtain and while we might have more guns we would definitely not have huge stockpiles of ammo especially if we are on the move/mobile. Also, working vehicles would be harder to maintain and many cars would be wrecked from day one of the apocalypse. 

Not trying to put a stick in your thoughts, just counterpoints.

[quote]Hm, what about a a year and a half?[/quote]

If you ask me, our group as it is (10 people is a lot of people to feed and keep armed) poses a fun challenge already. I think a year or so is a good challenge rating yet still allows moderate resources to plunder and pilfer.

As GM you have two choices to set up as our inital problem in the starting posts: is our enemy the infected or simply amoral humans? Meaning are we dealing with armed assholes or hungry ghouls?