This is a National Park that I took a hike in last week. It is just north of Seoul, you can actually see the entire city from this vantage point. That is only if there is a slight haze that day, since it is almost never clear enough to get any kind of view. The hike that I took was about 14 km long. But this is the first time that I've been on a Korean trail that actually had well marked trails.
This is just another view of the mountains to the north of where I was at the time.