Don't ride on PCH in Oceanside. Cross the Pacific Street bridge in Oceanside Harbor and take Pacific St almost the length of Oceanside until you reach Cassidy St. I think it's the 7th stop sign. Make a left, cross the tracks and turn right at Broadway. Go about 3 blocks and turn left on Eaton St. Go 2 blocks and turn right on PCH.
It is a 68 mile round trip from McGee Park (the park just south of the Army Navy Academy as you enter Carlsbad) to the northern end of Dana Point Harbor. That's a good turnaround point. There are bathrooms in the park and a coffee shop across the street. Or you can go one block further south, make a left on Grand Ave. and go into town for lunch.