I got it as well...slightly different method.
Same convertible...go down ramp take a right then your first left followed by another right (this is a dead end...also the place you get the maintanence key...the reason you go here is it spawns a ton of zombies right behind you). Turn around and take a right and another right.
At the end of the road there will be a white sedan...get in and go straight...all the way to a dead end parking lot (again this spawns massive zombies)...turn around and take your first right and then your next right...follow this tunnel all the way (it curves up and left) to massive ammounts of zombies and a truck...
Get in the truck go out the way you came...go all the way down this road until you come to a T intersection...at this point you can turn around and go to the exit ramp (up and on your left) which takes you back to the convertible...rinse and repeat.
A couple of notes...you want to hit as many propane tank zombies as possible and remember that you can look at your map if you get lost. This method left me about 35 hours (after starting at 72) left...in which time I got the Karate Champ and I would have got the Gourmet had the Pizza I had gone bad before the chopper came...
I am going to work on Saint and Census taker this weekend...