The route is simple. The easy way is to take the subway, and according to an online calculator, it takes 30mins and 1250Won and you feel lethargic and unhealthy after the trip. Let me show you another way, about 6 hours, free apart from some bananas, powerbars and 2 litres of water and plenty of patience. In return you will get some nice sceneries and fresh air. With all the nice mountains around Seoul, there's no excuse not to do it the old fashion way.
Other more reputable sites will tell you what is in Seoul Land. I've only been to the zoo and only because it was free that day. Will it be a surprise if I said there are animals there? In confined spaces. With plenty of noisy kids. Discovering that there are no birds to fling on catapults. So collect yourself at the carpark, run up Cheonggyesan 청계산 due east, come down where Dallaenae-ro 달래내로 meets the expressway, and then crossing some shops and farms, proceed up the second mountain Inneungsan 인릉산 and end up somewhere in Seongnam, close to the airbase and then run along the river up north to Jamsil.
For this trip, I took a backpack with some emergency kit and wore a Suunto Ambit. The screenshot above is what was recorded for the trip. The plots are quite rough but it should be able to hack together a plan for the trip. I find that the best map to use if you had a phone with you would be Daum or Naver since they have the map of the trails. Be aware that the maps are not entirely accurate, sometimes there are sub-path along the way that was created recently. Sometimes they are parallel paths and sometimes it looks like a whole mob went for a shit together and created a new path.
Now, to trace the path, take a metro to Seoul Land. Walk due south across the large car park, and run up to a slope where the cars enter and right behind the guard house there is a small path that branches out to the right. I didn't find it in the beginning. The first time I took this route I bashed myself out of the jungle using another path. This new one is more "official". What happens is that you walk along a nice concrete path till you get to an area that looks like an exercise area, but is actually the start of the trail, up the ridge of the hill which you will follow, and go uphill all the way to Cheonggyesan. For those that are a little more macho than others, you might want to try and run here, but I don't think you will last the first small hill. There are a few peaks here, the flatter ones happen to all have helipads on them, but the tallest peak for this trip (where the Cheonggyesan icon is on the map above, has a stone landmark, and of course, a stall selling alcohol. Like most others on this route.
Once I get to Gyeonbu Expressway, shops start to appear. This is a good place to refill your water, but then it will not be a free trek anymore. Up to you. You know your limits. This is also where you can pick up some outdoor gears, some discount shops or outlets are here, and of course if you want to hang around like the locals, you can have some barbecue and alcohol after a long hike. I didn't have too much time to stop, and I still had more than half a tank of water in my camelbak mainly because I have been drinking off two bottles of water on the side of my backpack. Still feeling strong, I walked through the village, I don't have the name of the village, but relied on Daum Maps on my iPhone to get me through to the next trail head for Inneungsan. On the map it does look like this second mountain is longer trek than the first.
It was a relief to get to the area called Sinchon-dong close to Seongnam. But the best part of the trek is behind me now, and the rest of the way would be just running on pavements and breathing carbon monoxide. What you enjoyed for the last 20km is now going to be given up in the 10km run to Jamsil from here.
So till another adventure...