I don't race, but I do live and Cycle in Socal.
I live in Rancho Cucamonga. It's in the Inland Empire, which is the non-desert, non-Mountainous areas of San Bernardino and Riverside Counties.
You can ride all year around here. There are organized centuries every month of the year except for december. The terrain can be varied. Being from Michigan, you might to get used to doing hill climbing, as a lot of the popular rides are in the San Gabriel Mountains. Try googling for "Glendora Mountian Road" and "Bicycle"
Mountian biking - I don't do much of that. Maybe we can get BCIPam to chime in, since she lives in OC and is an avid mountian biker.