As soon as you start, the first chance you get go to the parking lot in Leisure Park, skip Jeff and Natalie Meyer and go right down to the parking lot.

Get in the car go in the tunnels, make a right and take it all the way down.

Then when all you can do is make a left do a u-turn and take it all the way back.

Then after you pass the exit keep going, make the left go up the hill and take the working truck.

Then repeat. I killed 30,000 thousand zombies in one day in the game you should too it only took me 6 real hours to go through one game day.