Here is the best website for competitive cyclists in Southern California.
socalcycling.comsocalcycling.comOpen ↗
Training rides can be found listed here:
socalcycling.comfrGroupRides.htmOpen ↗
Southern California is a very large area with many alternatives for riding. It has been the winter season training area for most US pros as well as many European pros. Ulrich, Rominger, and A. Mercyx, have trained in and around San Diego. Von Bon, Berzin, Van Petegem and the Mercury team trained in the Simi Valley area (80 Km NW of Los Angeles) back in 2001. Armstrong and Ekimov trained in the Santa Monica mountains area and the San Gabriel Mountains this year (my backyard - my teammate ran into them on one of our favorite climbing rides) and US Postal holds their training camp north of Santa Barbara (300 Km NW of Los Angeles). Many if not most of the US based pro riders train somewhere in southern california.
The riding is as diverse as the terrain. I am willing to be a guide for you if you end up here. It depends on my recovery. I was hit by a car three weeks ago and my leg is broken pretty badly. I hope to begin riding again in early to mid January. I live in the Pasadena area (30 km north of LA) and I know most of the rides listed on the above website. I am a competitive Category 3 and Masters racer. I am not a consistant threat to win races but I can usually hold my own if I am in shape.
Lodging? Once again SoCal is a huge metropolitan area. Depending on your budget you can find something anywhere.
Hope this helps.