Getting back to Seoul from Inspire Arena after a concert

The nearest station is Incheon Airport Terminal 2 on the Airport Railroad, about 3.9 km from the arena. That is not a walk.
The last Airport Railroad train toward Seoul Station leaves at 23:50, the same on weekdays and weekends.
If the show runs late, that train is hard to make. Budget for a taxi or the resort shuttle before you go.
Staying near Incheon Airport removes the problem entirely, and if you fly out the next morning it is the easier choice anyway.
The venue page on this site lists places to eat and sleep nearby.