Upgrade the Mtb and get a road bike.
What trails are easily accessible depends on where in LA you live. You could have an epic ride just out the door or you may be 30+min drive away. The San Gabriel Mtns offer a shyte load of long grueling climbs and long descents.
The Santa Monica mtns have some decent xc rides and you can easily plan an all day epic ride or just a quick, fun out-n-back. Park at the beach, ride up the bike path, turn inland, ride into the hills, make a nice 30mi loop and then cruise back to the car along the beach and even go for a swim.
SoCalTrailRiders - Good place to find/set up local rides and trail info
SoCalMtb good for trail info
VenturaCountyTrails - Lots of trails are on the border of L.A. and Ventura and this is a good guide
MountainbikeBill - Great trail info and covers a lot more then just L.A.
OTB - Good, friendly bunch of riders that frequent the San Gabs a lot
North Ranch Mountainbikers - good, friendly group of riders with more of a social club feel to the group.
