I have three recommendations if you want to go cycling in the Samseong station/Co-Ex mall area.
The Han river cycle path is only a couple of big blocks away to the North. Alternatively another nearby option is finding your way onto the Yangjae Stream Cycle Path; thirdly there is the green space of the 1988 Olympic Park.
1. The Han River Cycle paths
From Samseong it should be possible to go about 500 m East and then join a stream heading North (Left coming from Samseong station) that joins the Han River. You can cycle as far as you want then :)
2. The Yangjae stream:The Yangjae stream cycle path is one area near to the Co-Ex mall. On this map if you went from the stop symbol to the play symbol you'd ride from near the Co-Ex Mall to the Yangjae forest. It looks about a 10 Km one way trip along the Yangjae Stream cycle path. Change the map from satellite to map mode to get some place names. This route should provide a pleasant ride to the Yangjae Citizen’s forest, which is apparently more pleasant than its rather stiff utilitarian name.
3. Olympic ParkLocated near to Jamshil Station (Subway line 2) Olympic Park (click for a google map) is a nice place to explore. There is a gallery, a museum, numerous waterways and sculptures, a historic site and cyclepaths leading North West to the Han River cycle way and South East to a cycle way next to a stream (according to the Seoul Bike Map).