The big event around these parts this weekend is the Komen Race for the Cure Saturday morning at Cal Expo. Registration is closed, but you can go out there anyway and cheer the racers on. Details here.
Another fun and challenging race that I've enjoyed (though my quadraceps complained) is the Mt. Tam Wild Boar Run 10K and 18K in Mill Valley on Saturday. Great off road trails highlight this run, not to mention cool T-shirt featuring snarling boars. Details here.
Triathletes can flock to Angels Camp for the 17th running of the Angels Camp Triathlon. It's a 1,00-yard swim (wetsuits allowed), 16-mile bike and 4-mile trail run. Details here.

