Zombies, RUN! iPhone app avail 2/27/12 "The premise is simple: you are "Runner 5" set out on a mission to collect supplies and accomplish different objectives in a post- apocalyptic world. The app takes you through 13 audio missions, all which feature an ongoing story that you are an active part of and that involves the occasional zombie chase." As reported on npr.org
Zombies, Run! 5k Training (C25K) Fitness Game | $1.99 USD | For Android (linked) and iPhone | The app I used when I realized I wasn't going to be able to go straight from a bed to Zombies, Run! ;) Similar UI, story, and feedback, but gives you walk periods, coaches you through stretches, and is generally better-paced for people just trying or reacclimating to running.