Ooo good luck! Marathons are addicting. Do you have a fuel plan? Also body glide. Everywhere. Especially if there will be any humidity or precipitation - it will make life more comfortable around mile 23.
Also if you're food motivated like me, it might help to have someone at the finish holding your favorite dessert haha. It's literally all I think about the last couple miles.
Hope it goes well! I'm sure it will be beautiful. Also have fun in S. Korea =)