Your not going to find challenging trails around here. Some of the seasonal/unmantained roads are challenging. You will need a county map and a day to explore if you wanna find them. If you are looking for challenging trails you should go to the Parks. Rocks and Valleys,TT,Bundy Hill, ect.
Be careful in the National Forest and always follow the map. There are Rangers out there and you will get a ticket if your not on mapped trails.