This is the "main street" of the park and the easiest way to get up into the backcountry with connections to all of the singletrack trails. Typical loops are heading up Lake Marie Road and down any combination of trails that run mostly parallel back to the parking area.
Although the start of the road is from the entrance going west, most riders take the shortcut from the day parking area south through the campgrounds. Just follow the signs to the trails.
Beginners may find the first climb a little challenging but there are rest benches along the way with nice views overlooking Napa Valley. The grade eases a bit after the first half-mile or so. It's also more shaded as the trail climbs towards Lake Marie where there's a picnic table.
This is the only trail in the park that remains open after heavy rain.