The Skyline National Recreation Trail weighs in at 12 miles with plenty of steep climbs and descents. There are lots of rocks in the more open treeless areas, but the trail smooths out every time you enter into the trees. Along the way, the singletrack intersects several other trails.
It can be traveled either direction, as both have tough climbs and steep descents. But, south to north as mapped is generally more fun. Whichever direction you travel, most runs will begin or end at the Skyline - Big John Flat Trailhead even though there is roughly a mile-and-a-half more trail to the west.