Bundang is south of Seoul and features a 3-loop system of trails. I found out the hard way that if you aren't used to climbing, you will pay the ultimate price. The approach to the trail is up a hill and you have to ride the first few KM on the highway, which switches to local roads, and ultimately switches to singletrack with steep climbs that you need to drag your bike up. Plenty of hikers are on the trails as well so you have to watch out.
If you are coming out here, join the Yahoo group for MTB Korea - they're great folks who can hook you up with local rides. Seoul is littered with bike trails all around on the outskirts and probably 100+ miles of government-sponsored bike trails all around the city.
The climate in Seoul is a cross of Austin's heat and Houston's humidity. Hydrate well or you will be very sorry. In the winter it is too cold to ride many days. This ride was in the 40's and sunny for mid/late November.
Shade: Heavy on the trails, light on the roads between
Post Ride Beer: Normally I would say a Cass or OB because I always drink the local beer, but we had Miller Genuine Draft, but when it is really cold out, remember to keep on glove on to hold your beer.