Training is important a a century is often compared to a runners marathon check out this web site:

ultracycling.com/training/century.html

The Amtrak is a great first time century. Well supported.
25 miles if irvine flats and hills, break, 25 miles of coast highway, break, 12 miles to lunch, 20 milea to Torrey pines hill, popcicle, coast to the train station. Remember PCH may be a net sea level ride but it rolls up and down quite a bit.
A good training ride is 62 miles and goes like this Anaheim Amtrak station (angel stadium) Santa Ana River Trail south to PCH - PCH south to Oceanside - Amtrak from Oceanside back to Anaheim. Very representative of the Century ride.
Happy Trails