November 2011 Update: A section of the trail between Georgetown Pike and the Difficult Run/Ridge Trail intersection is closed due to a washout. Experienced hikers will be able to negotiate the washout without difficulty, but it would be dangerous for inexperienced hikers and most children. I carried my bike along the rim of this washout, and it was precarious going and not something I would recommend.
The people in charge of "planning" in Fairfax County, a part of Northern Virginia close to Washington, D.C., bungled badly. Instead of having or requiring the infrastructure necessitated by growth, the government essentially let developers and builders tack up anything they wanted just about anywhere they wanted. Today, most of Fairfax County is densely populated, with new homes, offices, and stores still going up anywhere builders can find space for them, and traffic in the larger region each year gets the honors of being the second- or third-worst in the nation. It is also one of the most overpriced housing markets in the country, and you get a little for a lot when you buy.
Fortunately, the planners did have the good sense not to allow development in floodplains, and the county has several ribbon-like parks that follow stream valleys. One protected watershed is the Difficult Run one, and it has the region's best mountain-biking trail-- the Difficult Run Trail.
The Difficult Run Trail has one end by an office park in Reston and the other by rapids on the Potomac River in Great Falls Park. Great Falls Park is administered by the National Park Service and is a pocket of dramatic scenery and wilderness in an area that otherwise largely lacks them, and the park is the premier rock-climbing area in the Washington, D.C. metro area. For more information about the climbing and hiking opportunities at Great Falls, please visit this page on SummitPost.org.
For about 12 miles, the Difficult Run Trail winds northeast-southwest through the woods of Reston, Vienna, and Great Falls. Along the way, there are several good hills, rocky sections, and stream crossings. The scenery, especially in the many areas where the trail parallels Difficult Run or Colvin Run, one of its tributaries, can be lovely, especially in spring and fall. The other highly scenic section is where Difficult Run drops into a small gorge to meet the Potomac River.
The area is also a haven for wildlife. Watch for hawks, deer, and foxes. Also, watch for the little guys like chipmunks, snakes, toads and turtles on the trail.
Note: Some maps and descriptions have the trail beginning in Oakton and not going through the Reston woods and Lake Fairfax at all. This, though, is contrary to almost every guidebook, online, and spoken description I have ever had about the trail. This "other" Difficult Run Trail is part of the 40-mile Cross County Trail and follows much of Difficult Run Stream Valley whereas the "real" Difficult Run Trail first follows Colvin Run, and that might be the source of the discrepancies. In any event, this page describes the trail from Reston to Great Falls only. More about the Cross County Trail.
Most of the time, the trail is easy to ride, but there are some technical sections that earn the trail its moderate rating. The trail is, however, passable to hybrids, though the rocky sections are not much fun with one and can easily kill your tires if you go too fast. The parts of the trail that go through Lake Fairfax Park and Reston have the best and most challenging riding and are the most popular; in the Great Falls area, the trail is generally flat with some occasional easy rocky sections (the Great Falls section makes for very enjoyable riding if you have a hybrid, but it does get steeper and bumpier right near the end at the Potomac River).
Most of the time, the trail is a singletrack, but there are sections, especially in the Reston and Lake Fairfax parts, where the trail is wide enough to accommodate motor vehicles (this is necessary for maintenance crews to get in), though it is still a trail and not a road. There are a few spots where the trail is a closed dirt or gravel road, but those spots are very short. The entire trail is closed to all motor vehicles at all times except for maintenance purposes.
There are not, to my knowledge, any formally designated sections, but I have found four sections that are distinct from one another. Although plenty of people ride the entire trail in one outing, most people just ride a section or two, which is easier to work out logistically. My description is based on a trip from Reston to Great Falls.
This section begins at Michael Faraday Court in Reston and runs for about 1.5 miles to Lake Fairfax Park. It has some good hills, some interesting rocky spots, lots of exposed roots, and two stream crossings, both of which can usually be ridden.
Following the trail in this section can be somewhat confusing. There are numerous side trails, and most of them are dead ends or lead to neighborhoods. If in doubt, stay on the wider, better-worn trail.
There is one spot that is particularly confusing, though. About a mile from Michael Faraday Court, right after a stream crossing, the trail forks, and both branches are equally wide and well-worn (there are photos on this page illustrating this spot from both directions). To stay on the Difficult Run Trail, you must go right. Look for the posts blazed with horseshoe symbols. Through Reston and Lake Fairfax Park, it is these markers that indicate the correct trail.
After the junction, there are two pretty good hills, one of which can be challenging to ride up due to the number of large roots running across the trail, and another stream crossing before emerging at the soccer fields in Lake Fairfax Park.
Lake Fairfax Park
This section is only about a mile, but it is a fun mile, and it follows Colvin Run most of the way. Keep following the horseshoe signs and enjoy some short but steep hills and a few stream crossings. There is one short boring section as the trail breaks into the open and passes by some cricket fields, but it is otherwise a consistently good ride. This section seems to see less foot traffic than the Reston section does. Eventually, the trail passes by a large picnic area and leaves the park. Very soon after that, it crosses Hunter Mill Road next to a one-lane bridge and continues following Colvin Run through Colvin Run Stream Valley Park.
Colvin Run Stream Valley Park
This pleasant, mile-long section has a few interesting hills and some easy rocky spots but is overall dramatically gentler than the preceding two sections. Colvin Run is wider and more scenic here, and there are no stream crossings.
The section ends at a residential street called Carpers Farm Way. Here, it is easy to get on the wrong trail because the Difficult Run Trail now joins the Cross County Trail, which is marked by "CT" signs (follow these the rest of the way now-- one is pictured in the Getting There section).
To stay on the Difficult Run Trail, you must cross busy Route 7 (Leesburg Pike) to Colvin Run Road and the continuation of the trail by a gravel parking area. The stoplight at the intersection here may offer you your only chance of safe passage across.
Great Falls Area
Now the trail follows Difficult Run itself through Difficult Run Stream Valley Park all the way to Great Falls Park and the Potomac River. This is the longest and easiest section of the trail. The hills are very few and small, rocky spots are mild, and stream crossings are via concrete posts or placed rocks (the stream is typically too wide and deep to ride through in this section). There are a lot of muddy spots in this section, more than there are in the other three sections combined, and the muddy spots just get worse as people ride and walk around them, which often just expands the mud cover. There are some quiet street crossings, and sometimes the trail passes right behind people's back yards, but it is mostly very scenic and serene.
When the trail reaches a parking area on Georgetown Pike at a prominent bend of the stream, the county-maintained section is over. Technically, it continues for a very short distance before underpassing Georgetown Pike and entering Great Falls Park. From the park boundary, the trail has about another half-mile to go to reach the Potomac River. But the riding doesn't have to end out here; there are some other trails to explore in Great Falls Park. One of them is the Ridge Trail, which has one end on the Difficult Run Trail and its other on Old Dominion Drive near the park entrance station.
This is a mixed-use trail and is very popular. It is best enjoyed early in the morning before too many people are out. Later, you will be sharing the trail with hikers, dogs (often unleashed), and horses, which will reduce your riding fun substantially and dramatically increase the risk of a collision. Trail etiquette is for bikers to yield to horses and pedestrians.
Also, watch for deadfall, which has a habit of staying in place for a long time. While there doesn't seem to be much tension here between riders and other users, there definitely is a sense that keeping the trail obstacle-free for riders is not a priority.
Although it is not a formal rule, it is best, to preserve the trail, not to ride here for a few days after it has rained.
There are several places people use to access the trail. I will describe four in particular. These spots are all in Fairfax County, Virginia, and using a tool like MapQuest will help greatly if you are not from the area.
Michael Faraday Court-- This street is off Sunset Hills Road in Reston, the section of Sunset Hills between Hunter Mill Road and Wiehle Avenue. Currently, Michael Faraday is the only intersection along this stretch with a stoplight. If approaching from Hunter Mill, turn right; if approaching from Sunset Hills, turn left. Michael Faraday is a short cul-de-sac that at its end has two branches-- the left is a driveway for the Academy of Christian Education, and the right is the driveway for the Reston ice rink. As you drive down the street from the light, you will see a trail straight ahead going into the trees. That's it. Some people use a parking area across from the ice rink, which has a short spur trail accessing the main trail. Use this trailhead to access the Reston Woods section.
Lake Fairfax Park-- Admission is free for county residents, but I've never seen anyone checking at the entrance station, anyway. From Baron Cameron Drive in Reston between the intersections with Leesburg Pike and Wiehle Avenue, turn onto Lake Fairfax Drive and go into the park. At a stop sign by the picnic area, turn right and drive uphill a short distance either to the cricket fields or beyond to the soccer fields. In both cases, the trail skirts the far end of the fields. These trailheads are good places to access the Colvin Run Stream Valley Park, Reston Woods, and, of course, the Lake Fairfax Park sections.
Colvin Run Road-- This is a horseshoe-shaped road that intersects Route 7, Leesburg Pike, twice. Turn onto the road at its eastern end and look for a parking area on the right. The trail is easy to see. You can access the Great Falls and Colvin Run sections here, but it doesn't make much sense to use this solely for the latter. Crossing Leesburg Pike is dangerous, and it's better to park at Lake Fairfax if the Colvin Run section is your main interest.
Georgetown Pike-- Off Route 193 (Georgetown Pike) in Great Falls between Old Dominion Drive and Towlston Road, there is a large parking area on the south side of the road. Use this spot to access the Great Falls section. Be extremely cautious when pulling in and out of here, as the road is narrow and winding and visibility is poor.