Don't let the title "Difficult Run" fool you--it's named after the stream it parallels, not its level of difficulty. There's nothing technical to speak of here--unless you like the technical nature of erosion, washouts, mud bogs and consistent hoof damage. The mud bogs remain even long after any rain. The trail sees no maintenance, so the ravages of the traffic and weather go unchecked. The trail's not all like that--the western portions are better than the eastern portions. If the whole length was as good as the western end, I could give it three stars, but the east end barely rates one star. There are better trails in the area.
The Difficult Run trail actually runs through a large chunk of Northern Virginia suburbia. There are numerous access points as it crosses many roads. The easiest access point is the trail's eastern origin in Great Falls National Park. If you don't want to pay the park fee, you can park at the Difficult Run Stream Valley Park parking area on route 193 a little south of the park entrance. Riding south will soon deliver you into the park, where only the doubletrack is open to bikes. Riding north will take you into the length of the trail.
http://www.singletracks.com/item.php?c=1&i=2897