Unless I'm misreading something, the big rock is the beginning of the old trail (which is only less than one-tenth of a mile from the beginning of the current trail), and is not a section within the trail. If you are beginning your hike at 3-4am, then you will be at the big rock at 3-4am.
The big rock is the largest rock at the apex of the circle of the paved road at road's end as it circles back to head out again. Once you find the big rock, you enter the old trail by going around the left end of the rock. There is no obvious "trailhead" that begins right at the paved road; but once you are around the left end of the rock (a few yards off the road), you should see the path clearly as it goes along the back side of the big rock before switchbacking endlessly and steeply up the hill.
CaT
