I followed Christophe Stahl's description, running a counterclockwise loop beginning from Hahamongna Watershed Park. I ascended the Brown Mountain Road via the Fern Truck Trail and descended the switchbacks of the Ken Burton Trail into the Arroyo Seco. Climbed up over the little spur and plunged back down into the arroyo and ran the Gabrielino NRT back to the start.
This is a very fun and popular route among local trail runners in Northeast Los Angeles and the San Gabriel Valley. It's very runnable elevation gain. I have no doubt local speedsters will snag this one in no time flat.
This might have been the quintessential SoCal day to run this loop. We haven't had a hard rained in awhile, and the trails have been cleared of debris by the terrific stewards in our area. Temperatures were in the upper 50s to low 60s with a slight overcast. Can't beat trail running in L.A. winters.