Good concept! I thought about something similar recently, but your concept adds a few more ideas (e.g. running for your city / country). I also (or better: especially) like your way of presentation. Maybe it needs a few more details about how the app could look like and how the user interacts with the app (e.g. settings, selection of challenges, spoken updates during a challenge).
I think I'm going to scratch this concept. Even though it's not a bad ideea, I don't think it has enough potential to be great, as other apps are already doing a pretty good job at satisfying the runner's need for recording statistics and comparing them with friends. Adding this competitive edge over the other apps that are already doing a good job is not a big enough step to stand out. As you said, techincally simple but highly creative would be much better. I'm still thinking about how to turn this into something genuinely new or radically different.