One thing about moving through the woods is that you often come across what you don't expect. Add in the acceleration of a mountain bike, and a seemingly placid landscape--with dirt paths and a shaded forest floor--can startle you in a hurry.

I did not see the deer until I was a few feet away from it. I had rocketed down Old Carriage Road at Great Falls Park in Northern Virginia, pedaling until the wind streaming past my ears turned into a roar and the overhanging branches morphed into a green tunnel. Small stones pinged off the knobbed tires, and by the time I wheeled into a rut bisecting the trail, the image of Humpty Dumpty had taken hold.

Distracted by thigh- and lung-burn, but still upright, I gazed ahead and was startled by a deer, motionless just off the track. I stopped. It slid a few yards into the brush, but turned and glanced at me. After a few moments, it stepped onto the dirt path, a burst of sun lighting its flank. Unhurriedly, it receded into the woods on the opposite side, the leaf-crunch of its hoofs soon overtaken by the echo of barking.

"We see a lot of them," biker Meg Albus said after hearing about the tawny-colored beauty. "They're not afraid of you. They're pretty cool."

Deer are abundant throughout the Washington area, and so are good trails for mountain biking. You do not have to drive far to reach cared-for tracks of varying difficulty. On this early weekday evening I'd chosen Great Falls Park, which is run by the National Park Service, but the Louise F. Cosca Regional Park in Clinton or the Fort Circle Trails in Anacostia would do just as well. Spinning through a few miles of terrain can take the edge off a long day of work.

As Albus and I chatted in the unpaved parking lot at the Georgetown Pike entrance to Difficult Run, long a popular spot for off-road bikers, a stream of cars flowed past in the rush toward home. Albus, who is a Web master ("Sounds pretty authoritative," she joked) for a market research firm in Tysons Corner, had looked forward to this ride for hours. She would be joined by David Preznuk, who had shed his tie and dress shirt for a green racing jersey and biker shorts, leaving his 9-to-5 clothes in his 4x4.

"This parking lot gets really jammed on weekends," Albus said, although there were more cars now than when I'd passed by a couple hours before. "That's why it's nice to come after work. It's the only thing that gets me through to the end of the day."

Greg Porter did not have to wait that long. He and Chris O'Connell beat the rush by arriving at Difficult Run in midafternoon. Porter works at Giant Food in Fairfax, but not on this day. The bikes were unloaded from the car and the trails beckoned.

"I go out whenever I get the chance. We both got the day off and I figured, 'What the heck?' " Porter said. They'll bike for "a couple of hours. Maybe even more. Just let the traffic die down before we head back."

O'Connell, who started off-road riding two years ago, seemed prepared for his first trip on a track that is rated as "moderate" in Mountain Bike America's Washington-Baltimore guide.

"I don't know too much about the trails," said O'Connell, who lives in Fairfax. "It's the first time I've been out here. Nothing too hard-core, I expect. Hopefully."

There was nothing moderate about being looked over by a deer standing a few feet from you, or listening to wind whistle as you leg-piston deeper into the forest. Gliding past ferns and fallen logs and deep-shadowed undulations in Great Falls Park, it was hard to believe the city and its rush hour were just a few miles away. But that's part of the charm. With a mountain bike and an hour or two, you have access to things and places you'd never imagine.

Here are a few of the many area trails worth visiting:

Fountainhead Regional Park in Occoquan: Moderate to difficult. It is "hilly but a good ride," said Scott Adams, the editor of the Mountain Bike America guides and co-author of the Washington-Baltimore edition. "It's well-maintained and clearly marked. You're not going to get lost in the middle of nowhere."

Accotink Trail at Wakefield Park Recreation Center in Annandale: Adams rates the 6.1-mile loop as easy: "Nice place to get started."

Fort Circle Trails in Anacostia: A hill-filled, 12.2-mile round trip that connects several Civil War-era fort sites.

Louis F. Cosca Regional Park in Clinton: Easy to moderate terrain, but I came across some challenging hills on a recent visit.

There are several local mountain biking organizations, including the Chesapeake Mountain Biking Club (www.annapolis.net/cmbc) and the Mid Atlantic Off Road Enthusiasts (www.more-mtb.org/index.html). The International Mountain Biking Association's Web site is www.imba.com.