If you're staying in the centre of Seoul, I don't suggest taking the train to ICN (or GMP for that matter). To ICN, check to see if your hotel is served by the KAL Limo Bus (www.kallimousine.co.kr), which is probably the easiest/fastest way to Incheon (short of taking an expensive cab). To GMP, take a taxi directly from your hotel... It should cost around USD 30 or so, depending on where you are coming from.