Run the second oldest footrace in American, the scenic 7.1 mile Dipsea, from Mill Valley to Stinson Beach. The number of runners is restricted to 1,500 (to control the environmental impact) so you must register early and cross your fingers. To learn more about how to enter, check out the website.
If you don't get into the race, step it up a notch and enter the Double Dipsea. The course starts in Stinson Beach, runs over the Dipsea course back to Mill Valley at which point you turnaround and return to Stinson Beach. It is 13.7 miles of grueling steep mountainous trails.
Or...take the ULTIMATE challenge and run the Quadruple Dipsea. As the name suggests, it is the Douple Dipsea course x two. It is 28.4 miles of very hilly trails and with many stairs, including 9,200 feet of climbing!!