Bay Trail from Fort Baker

I haven't been there, but this is from the Wheelchair Rider's Guide to the SF Bay pp 12-13. She started from the west, but evidently found the Bay Trail steep but unobstructed down and under the bridge to Horseshoe Bay and Fort Baker.

Bay Trail and Battery Spencer
FOR AN EXHILARATING but challenging ride, and a different perspective
on the Golden Gate Bridge, you can't beat the stretch of paved service road
that winds from the west side of a trailhead parking lot off Conzelman Road,
just west of the bridge, steeply downhill and under the bridge to Horseshoe Bay
in Fort Baker. Part of the Bay Trail, the road is closed to vehicles except those
used for maintenance. But be careful: I went down only as far as the underside
of the bridge, and my power wheelchair worked hard on the return. For manual
wheelchair riders the continuous uphill climb would be extremely difficult.
A few hundred feet down Conzelman Road past the Bay Trail trailhead
lot is a roadside parking area for Battery Spencer. Two steep gravel paths lead
south from the parking area up to the battery, one of the many artillery em-
placements built to guard the Golden Gate in the late 1800s. One path has
steps on the approach, and during my visit the other had deep ruts that prevented
access beyond 50 feet. But even from the parking area you can look
down on the bridge and out to the ocean and feel as if youfre perched on top
of the world. Binoculars are a must here.
GETTING THERE From Highway 101 South in Marin, just before the bridge,
take the Sausalito exit and follow the Golden Gate National Recreation Area
signs. From Highway 101 North, after crossing the bridge from San Francisco,
take the Alexander Ave. exit.At the stop sign, turn left under the freeway, then
take Conzelman Rd. to the right (be careful not to get on the freeway here or
you will end up back in San Francisco). The trailhead parking lot is on your immediate
left after you turn onto Conzelman, shortly before Battery Spencer.
PARKING/RESTROOMS The trailhead parking lot has three blue spaces;
there are none in the dirt-and-gravel parking area for Battery Spencer. The
portable toilets at Battery Spencer are not accessible. The Marin Headlands
visitor center, three miles northwest on Field Rd., has accessible restrooms.