Go to a bike(one that sells mountain bikes) shop,ask if they still make "Franko's maps of the Santa ana mountian range". If so Buy one. He'll show you trail heads for fire roads(sounds like what you want). or go up to Ortega hwy. and stop in the ranger station, ask where the trial heads are.
There are many roads in the OC. you can go on. One dirt road will take you from OC to SD off of Ortega hwy.
Happy hunting! Have fun!
