Well, first this is a personal challenge, so I'm not seeking outside ideas. The goal is to help myself learn to design games. I'm always glad to help others though.
First, you've gotten a goal: Make running fun (a good one, I hate running just for the sake of exercise). But the PLAYER does not have one yet. What is going to make the player have fun while running? Seeing a 3D model superimposed over the image of themselves? Having the technical ability to do this does not make it a game or any other form of entertainment.
Now, if you had a system to translate the player's speed into some ingame state (speed, stamina, score, etc) that would be a start.
And additionally, design two is posted. Check out Cubicle Crop-duster!