I moved away from the bay area but I used to always make the effort to drive over to beautiful Sawyer Camp Trail where there is a great 6 mile out and back run (that's a total of 12 mi. for us mathematically handicapped). Best part of this run is the view of the surrounding mountains and the fact that the paved bike trail (although you will see runners cruising along on the dirt next to the paved trail) has mile markers at every half mile for you to pace yourself. It's a multi-use paved trail so you'll be sharing it with walkers, the occassional rollerblader and biker. There is also a decent small hill climb from mile 5 - 6. If you are in Silicon Valley its a great run to do if you can't find the time to get up to the Marin trails area.