Zombies, RUN!

Zombies, Run! works under the premise the city you're running across is filled with the brain-eating undead, and users are fed an audio narration revolving around post zombie apocalypse missions (collect water, food, medicine). It's one part audio book, another part video game, and a sly personal trainer, combining all three elements to create an entertaining workout which makes you forget you're actually working out.

$7.99 in the iTunes Store: http://itunes.apple.com/us/app/zombies-run!/id503519713?mt=8