There is something for everyone at the Duke Forest. Most of the trails are frequently occupied by casual bikers, some extreme bikers, joggers and labrador retrievers. There are some singletrack which is restricted to foot traffic only. Otherwise, there are probably 15+ miles of trails to challenge your speed and biking skills.
"From the Durham Freeway, take US 15/501 south towards Chapel Hill. Take the exit for NC 751 and turn right. Pull off on any of the gravel areas on the side of the road and jump on the trail at any gate you like.
http://www.singletracks.com/item.php?c=1&i=628