If you're in LA and lookin' for a good ride then consider yourself lucky. LA is full of amazing car-free trails to cycle up and down. Now when I say up, I literally mean up!
One of my favorite rides is out in the Angeles National Forest.
Your ride begins at the upper parking lot near JPL. From there you cycle through the Gabrielino Trail and up Fern Truck Trail until you reach the top. From there you'll descend down everything you climbed but with a little extra credit thrown in at the end. Follow my route and you'll be sure to have a great time.
I wouldn't call this a beginner route and I strongly encourage every rider to do their own research before being thrown into the mix. If you like climbing you'll love this route. If descending is your jam, then prepare to climb. As the topographic map shows, there is equal amounts of elevation gain and loss.