As I recall, it's very straightforward, save the part around Camp Pedleton. It's real easy to miss the turnoff (which will be on your right) that leads out of Pedleton and towards Oceanside. This turn off is to the "Old PCH". From what I remember, you go down a slight hill and after a small overpass, on your right will be some "parking" (there were cars parked here when I rode through) and just beyond that is the bike path. You follow that for a long while, and will lead you through the camp area and such. Once you get into the greater LA area, it get's a bit messier (if you continue through on the ocean-side bike path - [The Strand?]), but I guess that depends where in LA you actually want to end up.
Edit: Just realized you're going LA -> SD, so it's different than explained. Old PCH will just run into a road (after the fence and 'parking'), and you'll turn left.