Really? I thought you lost if you didn't finish the mission. That's the way it's kind of presented. I had no idea you could continue playing.
Same here, initially. I forget how I found out I could continue. I may have read it in someone's comment on some forum, somewhere.
What you say does make sense. We could really nail it down if we know when you had to finish "Backup for Brad" by. Like I said, it took me about 13 hours to kill 600 zombies.
What I'm wondering now is if you can get outside the mall though. If you go to the front where the zombies are coming in, you can go into in-between-the-front-doors area. At the front front doors there is a metal gate that has a big hole in it the zombies are coming through, with the parking lot and more zombies just beyond.
I didn't try it because I didn't want to risk dying when I was trying to kill 600 zombies. I doubt you can though, probably an invisible wall.
I don't think you can either. I tried. You can blast some zombies outside through the opening (if you have the Mega Buster), which gives the impression that it's a real part of the world, and you should be able to get there. Not so, apparently.
I had the Mega Buster and the laser sword (5-day survivor in infinity mode achievement), so I killed 600 zombies a lot quicker than 13 game hours. Same result. Should have paid attention to the clock, but I didn't.