Rained a little last night, and much of the park is still soggy prior to that. Get out when it's COLD. By 10 or so, it'll be greasy. Even at 28 degrees, there are still spots that are wettish
It's not going to stop people riding, I'm sure, but know what you're getting into.
https://www.strava.com/activities/2636849673/embed/f4d6fea0bbb2139bfc5e8b240b30d5adf0dc9401 starting at ~1:45 is the usual JORBA grand tour. The beginning loops out to Brisbane, and comes back along Tiger Woods. There are more trails both in/around Brisbane, but the one noted on said loop is the only legal trail. Use that information how you will. Across Hospital Road is yet more trails, but given that most of them are not marked, it's prudent to go with someone...not that you can get horribly lost, but still.
Parking can be done at the village or Mc Cann's (longish ride in/out), the tiny trail lot across from the water treatment plant/farm (have to be there EARLY to get there) with a shorter ride between both areas, or in the main Hospital Road lot.