Some days ago I met a beautiful place to train. It is about 20 min from La Jolla. It is a park called Mission Trails Regional Park.
There are several trails around there and a lot of mountains. Is perfect to do hiking and mountain bike. It's very big and you can ride for hours without seeing people, make sure to bring water and some food, you may need if you get lost. Try to go to the information center before adventure yourself. It is always good to figure out some information about the place.
If you hike early in the morning you can cross over the fog and appreciate a very nice view. The park also provide some events that you can join. Check the website for more information.
Good luck!!! Have a great adventure!!!