When it comes time to do long runs for a "mostly" flat marathon, you need a flat course. Most good running trails in LA are hilly. A terrific exception is the running and biking trail that follows the Metrolink Orange Line in the San Fernando Valley.
You can actually go farther than show on this map by continuing past the Encino Velodome and little league baseball stadiums through Balboa Park, then turn left on Balboa and right on Victory until you pick up the Orange Line path again.
One downside may be the many stoplights you hit. I like to run it early in the morning and look both ways before crossing. I also avoid parking in the Metro Orange Line lots, because their parking police are on the lookout for people who park but don't ride.
