I'm currently training for the upcoming Oakland Marathon. Last weekend I needed to go for a long run in spite of a steady rain. I decided to do the French Trail -- if there is any trail that will make a run good on a miserable day it is the French Trail. Here's the GPS. I was surprised to several other runners out that day, all covered in mud, and all enjoying the rainy day in the forest.
For those of you not familiar with the French Trail, it's located in Redwood Regional Park. The park has 4 major north-south trails that are excellent for running (East Ridge Trail, Stream Trail, West Ridge Trail, and French Trail). The French Trail is by far the most difficult, but also the best. I think it is the best running trail in the East Bay. The French Trail hugs the eastern side of the mountain that forms the Oakland skyline. Becuase of this geography, it is able to support the few remaining redwood groves in the area (these are second generation trees -- all of the old growth was cut down to rebuild San Francisco in 1906). The trail has several micro-climates with bits of chapparal intersperesed with redwood rain forest.
The trail is quite difficult to reach. You need to hike about a mile the trail head (park at the skyline gate, and take the West Ridge Trail). In order to reach the best part of the trail (between Tres Sendas and Chown Trails), it requires a several mile hike or run from the parking lot. The best way to access this part of the trail is to park at the Redwood Bowl parking lot, and take the West Ridge Trail to either Tres Sendas or Chown.