Happy Camp Canyon Regional Park in Moorpark is a 3,000 acre wilderness area with 12 1/2 miles of trails maintained by the Santa Monica Mountains Conservancy. I've hiked back here from time to time and rarely see anyone in these quiet canyons!
If you're looking for relatively flat trails well suited for comfortable hiking, mountain biking and horse riding, Happy Camp Canyon is a good choice.
You can take the main Happy Camp Canyon Fire Road for quite a ways and either head back the way you came or take the more strenuous Middle Range Fire Road back and catch some great views. I veered north up a steep maintenance road to some power lines shown below.
Or try the 8 mile, strenuous, South Ridge Loop (via the Middle Ridge Fire Road). Begin at the canyon gate; about 100 yards beyond, turn right and follow a dirt road up to the ridge top. Continue east along the Big Mountain Ridge to a junction, enjoying views on your right of Moorpark, Simi Valley, the Santa Monica Mountains, and all the way to Channel Islands. At the junction, turn north and drop down into Happy Camp Canyon. When you reach an old corral (and picnic area), turn west and walk back to the entrance gate on Happy Camp Canyon Fire Road (in the canyon bottom), approximately four miles.
NOTE: Because of sensitive habitat, dogs are not allowed past entrance gate.