The hills between Simi Valley and the San Fernando Valley are riven with lots of unnamed trails. In theory, they lie within the Santa Susana State Historic Park and Rocky Peak park, but in practice, they aren't maintained, are seldom traveled except by local kids looking for a place to party. There's an official kiosk at the trailhead, but after that, you're kind of on your own.
I hiked a nice loop through the area a while back, and last night returned with my running shoes. I'd originally planned to follow the path of my hike, but about a mile in, it was clear that the loop was far, far shorter than I remembered. So I extended it a little, and after pushing down an overgrown trail (and avoiding a tarantula that wandered past) I found my way toward the Santa Susana Pass Rd. onto a trail that looped back exactly where I needed to go. (Hooray for hiking with Google Maps in your pocket!)
I'm not sure I'll head around this particular loop again. Without the novelty of exploring these trails for the first time, it's just too overgrown and dirty to be really enjoyable. (However, the short loop I hiked last time might make a perfect 30-min run...)