Flight Incheon International Airport departs from Seoul Incheon International Airport at 20:40 and arrives at Tokyo Haneda Airport at 22:50.
The flighttime is about 2h 10m. The distance travelled ("as the bird flies"): 1487 Km. So the average speed is roughly 686 Km.
The best day to arrive at Seoul is Sunday and it would be best to avoid Wednesday, as prices are on average 140.6% higher than on Sunday. (Average prices, based on 2630150 datapoints.)
We have a lot more statistics for flights JL 5256 and between Seoul and Tokyo.