I did my first ALC having not ridden a bike since High School. I think the most important thing is that you have to commit to the time required for training. Training for me is one long ride on Saturdays (this weekends is 70 miles) and spinning during the week. As we get closer I'll start riding on Sundays. At the end of the month there's an organized double century ride where we ride to Santa Barbara, spend the night, then ride back.
You average about 80 miles a day. Some days are shorter, one day is 110 miles. The hardest part for me isn't the pedaling but being on the bike and in the saddle for that length of time. So for your first ride I recommend starting to train early and get used to being in the bike for longer and longer periods of time.
ALC has tons of organized rides every week and I'm pretty sure there's a group in San Diego. If you go to the ALC website they have a calendar of events
and you can satisfy your curiosity by taking a look at what training looks like now two months out.
On a personal note, this is my fourth time riding and it is one of the most amazing things you'll do on so many levels. It's one of those things that many people say "oh that's something I could never do", and IMO that's exactly why someone should do it. So I highly encourage you to stop thinking about it and commit to doing it next year!