South of Seoul near Kwachon, Chongye-san is a 10 km out-and-back along a ridge with great views of the surrounding countryside and mountains. The mountain bike ride starts with very steep climbs over the first 2km and is followed by fast, steeply-rolling singletrack.
To reach the mountain biking trailhead from Seoul, start from the Yangjae subway station (15 minute bike mountain bike ride from the Han River). Head south on Kangnam Daero Road which runs from Hannam Bridge to Yangjae Station. Just continue straight on that road out of Seoul, past Songnam until you see a stop-lighted intersection with a small sign on your right that indicates Chongye-san with an arrow pointing to the right (the sign might be in Korean only). Take this road for a while, heading west. Chongye-san will be on your right the entire commute. Youll go through a town about 2 km from the big intersection with the small sign. Continue straight through this town. A freeway will eventually end up in between you and the mountain. Just keep following the same road for another 2 km or so until you get to a free-standing gas station. This gas station is about 50 meters too far. Before reaching the gas station, there is a freeway underpass. Take it. Immediately on the other side of the freeway, turn left and follow the road parallel to the freeway for 100 meters or so. The trailhead is very, very close now, and might even be within sight. Looking at Chongye-san, you might see hikers wandering through a farm and into the woods to your left. Thats the trail, and you can access it from a few places right near that farm, which borders the road you just came in on -- the one that lies parallel to the freeway.
Start riding on the main ridge trail. You know you re at the top when you come to an overlook where you can peer directly down into Seoul Grand Park (look for the chairlift that crosses a lake). The mountains in front of you are Kwanak-san (a little to the left) and Umyon-san (a little to the right). Turn around and fly down the hill, which is what it really feels like riding on the ridge, with the mountain falling away on both sides.
Info courtesy of Steve Danyo