Wow a very ambitious goal.
First let me say this - years ago before I became a cyclist, I used to do the Lung Association rides that went from Tustin, to Lake Elsinore, down what is now the 15 and over to Carlsbad and back. I did that ride with no training. Did I suffer? Yes, but it was doable.
Riding along the coast is not all that flat. Many sections have large rolling hills. From LA to Huntington Beach it is flat (overpasses don't count) , with a few bumps but once you move out of Newport Beach and into Corona Del Mar, Laguna, Dana Point etc, the big rollies start. There are sections of flat but then you hit San Clemente (streep climbs on narrow, very busy streets), Del Mar (same as San Clemente) and then Torrey Pines. From Torrey Pines down into San Diego, traffic gets heavy and you must ride with caution.
Not certain where you live but ride for distance to get comfortable on your saddle and then once or twice a week do some hills. If you are anywhere near Palos Verdes - do the Loop. Or go from Pacific Palisades up to Will Rogers State Park and back. Also going through the Santa Monica Mountains or riding through the "Conejo" area like Thousand Oaks, Moorpark, etc. would be good training.
60 miles a day is very doable is you have a decent base which means at least 50 miles a weekend and at least some cardio traning (like spin classes) during the week.
Once the days get longer or if you can ride suring the day, then ride - at least 25 - 30 miles if possible.
Good luck - it's a fun ride. Just watch for traffic. Those crazy beach people you know! Also riding through the Marine Base - bring valid ID - it is checked - and also be aware that it is fairly hilly throough there. And then there are the tank crossings... :-(