PQ or Lake Hodges are both great beginner rides, anywhere with fire roads. Do an internet search of the major bike shops for “demo days” Bike Bling and the Trek Superstores often have them at PQ and Lake Hodges. Also, check Orange County, they have way more big bike shops that do demos and areas like Alisio Woods would be a great area to ride around a demo. Typically you can arrange in advance and if you wanted to ride for a few hours the demos might run you $50; however, if you play your cards right they will usually let you roll around for just your drivers license if you find them on-site.
Mountain biking is fantastic even if you never venture more than the fire roads, there are miles of open space in San Diego and around the country that is beautiful to explore on a bike. Load up the camelback with water and snacks and it doesn’t get much better.
I wouldn’t worry too much about flats with the type of riding you will be doing, DO NOT over inflate your tires and once you start riding worry about a spare tube and patch kit. My husbands gets flats/blow outs, but he is an idiot.
As for dangerous, I have a broken pinkie and a cracked rib from a fall last month, broke my arm twice…love it!
There are also a couple different places that provide bike skills clinics and tire changing clinics, the REI stores put them on fairly regularly, heard they are pretty good.