Seoul's transportation is one of the best in the world. With a population approaching almost 10 million people, the city needs an efficient, organized network for making sure everyone gets where they need to go in time. For a lot of people, traveling can be very stressful, but in Seoul things are pretty stress-free. Every sign is translated in Korean, English and Chinese, every exit is marked clearly with landmarks, and transfers are easy to understand. Even on the trains, there are arrows to tell you from which side of the train car you need to exit.
In Seoul, most people take public transportation. Nonetheless, people do have their own cars, and with that comes an insane amount of traffic at just about any time of day. Rush hour in Seoul is an exercise in patience, as you shuffle through the subway station, stand in an orderly line to get on the next train and then pile into the train car like a can of sardines.
Personally, I have been surprised at how much walking I have done so far in Seoul. It's really not uncommon for my friends and me to walk a mile or two to get somewhere. I imagine, however, that as it gets colder, our walks will start to get a little shorter!