Once early in my bay area hiking career, I tried to find the western end of Dias Ridge Trail, a fire road which drops from Shoreline Highway to Muir Beach. I was unsuccessful so I went on to explore some of the many other options nearby, including Muir Woods, Mount Tamalpais, and other GGNRA trails through the Marin Headlands.
But now I have a reason to go back and try again: Dias Ridge Trail has been rebuilt. Just over 3 miles, the fire road is an important connector for several area medium to long hikes. String it together with Redwood Creek Trail and Miwok Trail for a 6 mile loop, or jump across Shoreline Highway on Miwok into the headlands and tramp around all day.
Check the trail map here.
Muir Beach is a good trailhead; it's about 1/4 mile to Dias Ridge Trail and provides plenty of parking, plus beach access and food at the Pelican Inn.