Keep well motivated. For some people that means a rigorous regime and close monitoring of all bodily functions carefully plotted on spreadsheets. For others is means ride when you feel happy. Ride when to feel down, to get happy. Ride to work and to the shops. Get a map and explore all the quiter local routes. Ride with buddies who ride at your pace.
I find that most of my leisure rides are 15-30 miles , if do this regularly, you can easily double that distance for an event. You dont need to ride fast or had to get in shape for a 50, just get some decent saddle time. I tend to guage myself by time rather than distance or speed.
Do make sure that your bike fits you well, is reliable and properly geared for your terrain. Dress for the weather. In S Cal that should be easy. Winter is a more comfortable season than summer. Learn ro ride safely on the road, and take any food and water you need. Learn some simple roadside repairs and carry enough toolkit to get yourself out of trouble.