. . . includes Portland:
If jogging alongside Olympians and marathon champions is all the motivation you need, take a jaunt around Portland. Not only does the area serve as home base for renowned racers such as Kara and Adam Goucher, Shalane Flanagan and Dathan Ritzenhein, the headquarters for Nike and Adidas are also here. While downtown Portland has a decent path by the Willamette River, the real appeal is the slew of forest trails, park paths and routes just outside of the city center.
(The other cities are Chicago, New York, Boulder, Washington D.C., San Francisco, Austin, Boston, and Atlanta.)
I certainly notice a lot of runners in the Portland area, although maybe that's just because I'm attuned to running now. I'm not that surprised that Los Angeles doesn't make the list, but I am a bit surprised that San Diego isn't. Carlsbad is probably too small to have merited consideration, but when we were there last winter break, I definitely saw a lot of runners (and bikers).