Over on FlowingData, Nathan Yau pulled public data from the tracking site RunKeeper.com to map out the common running routes of twenty major cities. The results are neat to look at, but it's also interesting to see the park systems, waterways, and urban grids of these cities come to come life.
A stylized Washington D.C. is above (up is west, FYI). Here's Minneapolis:
It's important to remember that this data is biased towards people who would use an app like RunKeeper, so you can imagine that it's not representing a perfect demographic sampling of a city's runners. Another quirk is that Yau did not distinguish between organic runs and runs with set courses, so some of the major paths are clearly marathon routes.
Here's New York; people don't generally run up 4th Avenue in Brooklyn like that, except for that one day when 48,000 people do (the New York map in general is a little sparse/goofy):