The MMWD will ticket you for running your dog off-leash and the Mt. Tam SP and GGNRA will ticket you for having a dog on leash off, off-leash or a picture of a dog in your pocket.
Your best bet is Marin County Open Space lands. On Open Space areas dogs are allowed off-leash on fire roads and on-leash on trails.
If you are driving through Marin on 101 your best bet is running in the Blithedale Summit preserve, starting from the Corte Madera side at the end of Southern Marin Line FR. (The attached map makes it look like there is no parking here, but there is a small lot where you can usually find a space on a weekday and often on weekends. You can run a flat road for close to 3 miles each way, or go up to the ridge all on dog running wild legally fire roads. You can also run two great, but steepish trails, hoo-koo-e-koo or huckleberry on leash. (Actually I run these with my dog off leash all the time, there are few rangers in this area, but don't complain to me if you get a ticket.) A popular 5.9 mile run is go down to the fire road to the end and up hoo-koo-e-koo, then go left on Blithedale ridge road to Corte Madera Ridge road and down huckleberry to where you started. You can extend this by running further on Blithedale and coming back via Warner Canyon. Check out the map below.