I am not in Korea anymore, alas.

I used to live in Ch'unch'on so all my riding was in the area around Ch'unch'on, Hwach'on, Hongch'on, and Kap'yong. The roads are pretty good and there are some fabulous climbs/descents.
There's even a velodrome in Chunch'on and last I remember, there was a Kap'yong high school with a cycling team.