We are heading to Busan after a few days in Seoul as our family’s dying to see us :p just kidding XD My mother kindly got 2 first class KTX tickets from Seoul to Busan for us as all the domestic flights need to go through Gimpo airport, not Incheon airport (ICN).
You can still take a subway to get to either Seoul station (KTX) or Gimpo airport. Well, it depends on where you initially depart from, but if you stay near central Seoul city, then the KTX might be a better option as it doesn't take much time to get to the station. Although it takes about 3 hours to get to Busan, it's still pretty good as it's less stinky, has better leg room and free WiFi, all of which are pretty important to us.
KTX also has giant windows all along the side of the train, so you can enjoy the view as you travel. The seat has a pretty sturdy big table as well so you can actually do some work on a laptop too.
If you like reading, they have free books, magazines and newspapers to read. There's a shelf next to the entrance door, so just feel free to take it and just put them back before leaving. You also get a mini bottle of water for free, so make sure you get one before entering :).
My man and I've been eating Phad Thai noodles that we got before boarding, watching YouTube videos and doing work on our tablet and laptop. Surprisingly neither of us got any motion sickness like we do in cars, which was a relief.
Oh, by the way if you plan to buy some tax free goods at duty free shops, you should check their magazine as it contains discount coupons that you can use, as long as no one has already taken it ;). It comes in handy if you are a big shopper like me :p.