Yeah if you can evade the basic zombies through the first few levels well the game can definitely be done in under 2 hours. The runners get dropped easily by the upgraded pistol and the gators®enerators die to the shotgun easily enough. Ammo isn't a problem once you reach the army base levels and you should have enough pistol ammo to get through the city if you didn't shoot too many early on. IMO the only really difficult/time consuming parts are the ones in the dark, but maybe 75% of the subway can be skipped so it's not an issue. The park definitely gave me trouble when I was just running through it, and hitting the dogs in the dark is easier said than done. However, if you're really struggling with it or want to save ammo and your infection is low enough you can clear the area by just sort of just running forward, taking a few hits (the dogs die after they get you) and then going back to regen since Sofia's side mission is pretty easy and it gives you a full infection removal.