I love to run, and don't get out as much as I'd like, but a tool I love for calculating distances and planning my running or biking routes is Runningmap.com. I can plot routes pretty much all over the world and save them, and even make them public for anyone else with the same modest endurance ambitions. There are other, perhaps more sophisticated sites, but I've been using this one for at least 4 years and it's fine by me.
Via Good Magazine:
Coolest bus stops
Find walkable communities, and rate your own by determining your neighborhood's Walkability score.