JustMe

Sounds like a good plan. You can take the train all the way to Anaheim, and use the SART to PCH just north of Newport Beach, or start further north and use the LA River Trail to Long Beach and pick up PCH about 5 miles south at Seal Beach, or use the SGRT and connect via various routes in the north, and pick up PCH in Seal Beach. It depends how many miles you want to make the trip. There are several variations off of PCH that can add interest and avoid traffic, like using Bayside in Newport, the State Park in Dana Point, and going through the Marine base at Ocean side. If you want to avoid actually gong into SD, Solana Beach has very nice overnight facilities, restaurants, and a quaint Amtrak station.

You can find a number of threads on this, and other forums, for rides using each of the foregoing. If a gpx or tcx file would be of help, I'm sure those are available as well.