We have some stunning MTB riding around Los Angeles. One of our favourites is the Ken Burton Trail. Closed due to natural disaster and subsequently over-run by nature, the trail was reopened in 2016 after much hard work from the MWBA and CORBA.
Rolling out from the Windsor Car Park at JPL, the 'Ken B' is an intermediate to advanced 14 mile loop with 2000ft of elevation that tests both fitness and technical skills. See route here.
You can read more about the trail itself, and the trail work required to bring it back to life here.