Just 20h of "go from point A to point B and kill everything along the way".
Did I mention that you are immune to the zombie virus for some reason that is never EVER explained? There is no explanation for that, you just happen to be immune because it was probably easier for the developers/writers.
The difficulty varies so much that it is either a no-brainer to kill a thousand zombies or it makes it practically impossible to figure out who is shooting you. Co-op is the only suggestion in some of these cases. And due to collision detection and physics problems, soldiers usually got stuck inside walls and could kill me from both sides... But I couldn't kill them from any side. Also, when you hit anything, your weapons wear out. However, I have never seen such frail weapons. If you touch (even slightly) an enemy, your weapon loses ~10% of durability. If you upgrade your weapon to the max, it lasts a little bit longer, but you end up running from workbench to workbench all the time.
Really boring game.