If I have anything more than 10 hours. It is an hour long train ride into the city and an hour back.
Some FYIs…Google and Apple Maps work great from a go from X to Y place but the GPS portion does not work in Seoul because it’s so close to the Demilitarized Zone and the S Korean government doesn’t want Pyongyang to come rolling in with tanks using Google Maps to get to the capital and seat of power. If you can follow directions though, you’ll do just fine. Think Mapquest.
Much of the things you’ll want to see are very walkable.
Enjoy your time.