South of Seoul, Umyon-san is the most popular mountain bike ride in Seoul and probably is the only course 100% mountain bike rideable. If youre staying in Itaewon or in the immediate Kangnam area, this is the most easily accessed trail network. Its mostly undulating singletrack, with few rocks and none of the dramatic granite outcroppings featured in almost all of the other mountain bike rides in the Seoul area. There are about 20km of trails in the network, although it ll be next to impossible to find a coursing that includes all of these trails without a guide who knows the hidden trailheads and connectors (through backyards, farms, offtrail, etc.). In general, each separate trail is one or two km up, then one or two down.
To reach the trails from Seoul, head south towards the satellite city of Kwachon. The most direct way to get there is to first get to Yangjae subway by bike (a 25 minute mountain bike ride from Itaewon). From Yangjae station, head south on the 10-lane road, toward Kwachon city (past Yangjae station). Just past McDonalds, about 100m from the Yangjae station entrance look carefully on your right. Youre looking for a little driveway that dead-ends in a forested knoll. If theres a small trash dump around the corner and some stairs next to it, youre at the right place. Go up the steps (8 or so) and you should be on a ridge singletrack.
Ride along this, staying on the ridge. It will undulate for a short spell. After about 800m youll pass an exercise area, followed by a grave bordering the trail on your right (Korean graves look like a breast-shaped mound with a surrounding dirt wall). Take the next singletrack on your left, which should be 30m or so later. This should be headed slightly downhill. Then, after a few meters make your next right. At the bottom of this track, after 50m or so, youll be at a four-way intersection of paths; continue straight, uphill. Continue along this until you hit a major road. Turn right onto the road and pedal until it ends in a T-intersection, where youll turn right. Make your first left into an apartments driveway, but instead of actually entering the apartment, continue up the driveway to the next traihead. Continue uphill on the trail, where youll very soon come to a series of steps where you can mountain bike ride up the side. At the top there is a 4-way intersection. Turn left, up a short, steep pitch, and continue along this ridge until you hit a paved road or another major street. In either case, turn right onto the major street and mountain bike ride for about 1km. Look a minor paved road on your right which starts heading uphill, in between a woods and greenhouses. If youve arrived at metal barricades and a guard shack on the major street, youve gone much too far. This minor paved road will magically transform itself into a dirt road after 400m or so; you might have to search for this dirt road a bit.
Now the main trail network begins. There is a singletrack that parallels this dirt road. Also, about halfway up there is a 4-way intersection a few meters off to the left of the dirt road. The trail going downhill splits in two around the bend and after about 1km both trails spill out on or near the major road you just left. Follow your nose, it always knows. Almost at the top of the dirt road will be a short, sharp track on your left up into woods. Take this all the way to the true top, which some people consider to be a lookout. From here there is a trail straight ahead that immediately goes straight down steeply. It can be somewhat difficult or exceedingly difficult, depending on the time of year. Alternately, you can turn left from the lookout and follow this second trail down the other side of the cliff, which is what it wil become just beyond your sight. It is mountain bike rideable despite appearances, but requires some nerve, skill, and a very low saddle. It connects with the 4-way intersection you passed coming up the dirt road. Anyway, back to the straight-ahead, straight down trail: this is the Main Ridge Trail. You can follow this one all the way to the bottom, come back up, and go down the other trails you just passed. At the bottom of the ridge trail, youre in Kwachon city, and fairly far from Yangjae station.
The best way to get back to Seoul is to follow your nose downhill and out of the park area to the major road, turn right onto it and mountain bike ride up less than a kilometer to a huge intersection, with another major road going uphill on your right. This road is the one you want to take; it leads to the border between Seoul and Kwachon, a minor pass called Namtaeryong which everyone knows (in case you get lost). From Namtaeryong, descend the major road to wherever in Seoul you want to go. To get to the Han River, just go straight for 20 minutes or so.
Info courtesy of Steve Danyo
Overall, Umyon-san is the 6th most popular mountain bike trail of all 7 mountain biking rides in Korea.