There's miles of wild hills north of Simi Valley, but few marked trails since the area's mostly still owned by oil and gas companies. We did some exploration on our bikes yesterday, and found a challenging 9.5 mile loop that heads through there. It starts off near the north end of Yosemite Avenue, winds up along the Los Lllajas fire road, switchbacks up a hillside, connects with Rocky Peak fire road, and you can then take Chumash single-track back to your car. Here's the map.
From the 118, take the Yosemite Avenue exit and drive north for about 1.5 miles. Turn right onto Evening Sky drive and go about another half-mile until you see a gate across a fire road on your left, together with a small information sign. Park on the road, there is usually plenty of room.
Take the fire road for a short distance until it joins the main Las Llajas route. At this point it's paved, though inaccessible to vehicles. Turn right and head down the hill. You'll now stay on this road for around 3 miles, as it winds along by the creek. It climbs gently, and there's some good shade from oaks along the way when you need to rest. About 2 miles in there's a first gate, which should be unlocked but kept closed.
At the 3.5 mile point, you should see another dirt road heading off to the right. If you miss that, you'll come to a second gate festooned with 'Keep Out' signs, the entrance to Las Llajas ranch, and you'll need to head back a few hundred feet.
This dirt road climbs up the hillside and links up with Rocky Peak fire road. A little way up you should see an old oil pump. Checking the gauges it seemed like there was still pressure underground, and it seems in good shape. It's around a mile up to Rocky Peak road, and I found it a pretty brutal climb, especially in the sun.
When you reach the top of the ridge 4.5 miles in, the road connects with the Rocky Peak trail. It looks like you could head towards Oat Mountain if you turned left, but to loop back to the trailhead turn right. There's a short climb and drop, and then a long, tough slog to the summit.
Chumash single-track starts on the right side of the Rocky Peak fire road a ways after the summit, around 6.2 miles from your start. There isn't a sign, but there is a large charred post with some illegible blue paint on it, and the trailhead itself is fairly obvious. It's about 2.5 miles long, and the first half is fairly technical, with some tricky rocks to hop around. You'll drop down into Chumash Park, and there's lots of different variants on the main trail in there. Most of them take the same basic route, but you should watch out for a small side-trail heading to the right when you're nearly back to the main Chumash trail head at the end of Flanagan Drive. Take the side-trail over to Evening Sky Drive and head half a mile back to your car.