You can't do much of anything like that with Open Space and MMWD looking over your back.. Just noticed a new sign at bottom of Deer Park Fire Road in Fairfax stating that they are stepping up enforcement of dogs off leash, not to mention the disdain for Mt bikers on top of having a dog off leash.. Maybe some city park areas, but be careful!
Agree with the comments here. I keep my dog on leash until I'm a good half hour hiking time from any trail head. Most of the walkers don't go as far as a bike will.
One thing to keep in mind when you're riding with a dog; your dog is usually trotting along or running and this excites most other dogs on the trail so you have to be ready for that. With a leash I can pull my dog back behind my bike and I use the front wheel as a blocker. My dog, Eddie will bite back when a dog runs up onto him and sticks it nose in his azz.