Yeah - I'm one of the lurkers Gern mentioned but I don't have a brilliant suggestion so I didn't reply. I rode from Overland & Palms to Mission Bay and then up into Santee about two years ago. The PCH is the way to go, with some mild wrinkles, south of Long Beach. I think heading inland to pick up the LA river bikepath is too much of a detour. You might try amalgamating one of the routes suggested in this thread:
http://www.bikerowave.org/forum/index.php/topic,154.0.htmlwith a PCH route south of the LBC (yeah you know me!)
Alex