I purchased it after you mentioned it on the marathon thread. I haven't made an account yet on ZombieLink, but I plan to mess with it some this week, so I'll look for you.
I really like it!

We've just done two of the missions in season 1 so far, but it makes the run go really fast. We ran yesterday with another person who hasn't run with DH and me before. I was wearing the iphone, so it really only mattered how fast I went and on the last zombie chase I left DH and our friend in the dust.

She thought I was crazy. We were trying to sort of figure out just how much we needed to speed up to escape the zombies. DH thought we only had to speed up a little, so on the last chase I tried that, but the beeps and moans got really loud, so I freaked out and took off. I was so relieved (and tired!) when it finally told me "Zombies evaded." Phew! Anyway, I recommend it as well. We've also found our overall time has been faster and I definitely get a better cardio workout during those sprints. I wouldn't want to use it for my long training runs, but for my shorter midweek maintenance runs, it's great.
As far as how long they last, you have a choice in the setup to make them either 30 or 60 minutes. We've been choosing 30. The story segments are the same regardless, but in between the story segments, the app plays your music from a selected playlist. (I created a special playlist just for Zombies, Run). Anyway, it just uses more of the music in between story segments if you choose 60 minutes (or at least I assume that is what it does). And, even once you complete the mission, the app switches to what it calls "radio mode". You then get the radio operator who sort of acts like a DJ and talks some about Abel township (the home base for your operations) while playing more of your playlist. I didn't realize though when we first did this, that you will continue to get zombie chases while in radio mode even though the official mission is complete. You also continue to pick up supplies. So, if you don't stop the mission, be prepared for that!