At least not very hard. No matter what happens, there's always a bathhouse and food nearby. Although these eleven points of advice might scare some readers off, I should add that in 4 years of riding in Seoul and 4 years of riding in Atlanta, I'd have to say that I've had many more life-threatening incidents with motorists in Atlanta while riding bicycles, including having a gun pulled on me, having things thrown at me, etc. On to the trails. In Atlanta, I used to dread loading everything up in a car for a two hour drive to Appalachia.
Most trails are steep and many are very rocky.
To ride in Korea you've got to learn how to ride up and down steeps. The only exceptions to this are the Umyon-san network, and parts of Namhan Sansong, which are still very hilly with a few steeps thrown in to keep you honest. Umyon-san is also the closest serious riding area to Itaewon. All three areas are more ambitious than Umyon in terms of needed time, skill, and fitness. You simply must avoid Kwanak and Acha on Saturday afternoons and Sundays, due to hikers.