Zombies! Run!

I wish I still ran regularly so I could play this game/app: Zombies, Run! It's an app for your phone that works with the GPS feature to track your run distance, and is paired up with an online game. As you run, distance can be used to get medical supplies, ammo and other things for your group of survivors...apparently there are downloadable tracks where you can build your own running soundtracks interspersed with "news" stories and information about the zombie apocalypse or (eep) be chased by hordes of zombies.

Why are there not more interactive fitness games like this? Why can't you have fun while you're active? This is freaking awesome. I may have to start a weekly run/walk just to try this out and hope my knee is agreeable. I have learned that I am apparently getting an iPhone to go with my shiny new job...so I should make sure I know the complete functionality of it shouldn't I? You know...for science? :)
I always joke that I stay fit so I can outrun the zombies when they come...this is the closest I may get...