This loop is about 16.5 miles long with about 3,154 ft of elevation gain. It is designed to cover most of the Topanga State park trails and the most interesting sites like Eagle Rock, Hub Junction, Santa Ynez Canyon, and Temescal Peak. This loop includes a hidden trail on the north side that goes through 2 of the highest peaks in the area.
I counted 4 spots to park a car:
1. South West side: The Trippet Ranch at 20828 Entrada Rd, Topanga, CA 90290. $10 for the day. Open only from 8am to 6pm daily.
2. South West side: Just one block before the Trippet Ranch on the street, free parking, on Entrada Road.
3. South East side: Michael Lane, close to the Trailer Canyon Trailhead, free parking (around 1798 Michael Ln, Pacific Palisades, CA 90272).
4. North side: Marvin Braude Mulholland Gateway Park, 3600 Reseda Blvd, Tarzana, CA 91356. Free parking below the sign.
The description below is counter-clockwise starting from Trippet Ranch parking lot. Go up Entrada Road across the bathroom. Make a left up on Trippet Lane. Make a left up on East Topanga Fire Road. Go down the single lane trail called the Santa Ynez Trail. There is a very nice downhill with large rocks on the left and right. Follow the beautiful trail at the bottom of the Santa Ynez Canyon and exit the park making a left on the paved road Vereda de la Montura. If you make the loop clockwise, make sure to stay all the way on the left after the fence at this spot so that you don't get lost.
Continue this 0.4 mile section of paved road with a left on Michael Lane and go back on the trail with a left on Trailer Canyon Fire Road. Make a left on Temescal Canyon Road and go for about 4.5 miles. You can make a quick stop at the top of Temescal Peak that is at the corner with Temescal Ridge Trail. Go down the Bent Arrow Trail for about 0.5 mile. Here you have the option to make a left on Mulholland Drive to go back on Temescal Canyon Road, but there is an interesting hidden trail just before Mulholland up on the left. The Bent Arrow Trail just got enlarged in the area so you have to climb up about 3 feet where the hidden trail got cut. This 0.7 mile section is pretty tough on non maintained outgrown trail, but is completely worth it since you'll have the best view on the south side and the Valley from the highest peak you'll reach. If you don't feel like it, you can just go around it in the back going up the easy Mulholland Drive on the left, which will make pretty much the same distance overall.
At the bottom of the hidden trail make a left on Mulholland Drive for a very short section and make a left on Temescal Canyon Road. At the bottom take the Garapito Trail down on the right. At the end of this single lane trail make a right on East Topanga Fire Road where you'll see Eagle Rock on the left. At the bottom of the road take the single lane Mush Trail. After 2 miles, cross the Prier Road, and continue on the Mush Trail until you reach the parking lot at Trippet Ranch.
Got a long run today, fitness is popping, gonna see what I can do on this route or if I get lost. Expecting ~3-3.5 hours.
For anyone interested in attempting the route. I feel Christophe's description is unnecessarily complex, so here's route outline:
Trippet Ranch -> Santa Ynez Trail -> Michael Lane -> Trailer Canyon Fire Road -> Single Track Connector -> Mulholland Drive -> Temescal Canyon Fire Road -> Garapito Trail -> Topanga Fire Road -> Musch Trail -> Trippet Ranch
Any deviation from that route is optional and will probably cost you time if you care about the FKT.
It's a loop, so hit those landmarks and you'll be golden.