Flight Incheon International Airport departs from Seoul Incheon International Airport at 08:25 and arrives at Tokyo Narita International Airport at 10:50.
The flighttime is about 2h 25m. The distance travelled ("as the bird flies"): 1555 Km. So the average speed is roughly 643 Km.
The best day to arrive at Seoul is Tuesday and it would be best to avoid Monday, as prices are on average 85% higher than on Tuesday. (Average prices, based on 6931851 datapoints.)
We have a lot more statistics for flights OZ 102 and between Seoul and Tokyo.