It's called Zombies, Run! (App Store Link), it's a little on the pricey side for an iOS game, at $8, but you might find it worth it if you are in need of a reason to walk, or even run.
Think of it as RunKeeper but with zombies and up to 30 missions thrown in for good measure.
The just launched iOS version comes with 13 missions, and 17 more missions to be released with upcoming free updates. Zombies, Run! requires requires iOS 5 or above, and works on iPhone 3GS, iPhone 4, iPhone 4S, and iPod Touch 3rd and 4th gen. [Naturally something like this on iPad doesn't really make sense.]
To play the game, you fire up Zombies, Run!, plug in your headphones or connect your BlueTooth headset, and start running (or walking) As you go, you’ll hear a narrated story of a zombie-infested, dystopian future, written by award-winning novelist Naomi Alderman.
You play as Runner 5, a “runner” whose job is to venture out and collect supplies such as ammo, medicine and batteries for your base of survivors. The more you run, the more supplies you’ll acquire.
Once you return home, you can build and grow your base with the items you collected during your run.
Android lovers, be warned: your stint at exercise is coming to meet you: Zombies, Run! is coming to Android, as well.
Watch a demo video below.