Flight Gimpo International Airport departs from Seoul Gimpo International Airport at 16:20 and arrives at Tokyo Haneda Airport at 18:35.
The flighttime is about 2h 15m. The distance travelled ("as the bird flies"): 1450 Km. So the average speed is roughly 644 Km.
The best day to arrive at Seoul is Sunday and it would be best to avoid Thursday, as prices are on average 46.62% higher than on Sunday. (Average prices, based on 1506455 datapoints.)
We have a lot more statistics for flights JL 5236 and between Seoul and Tokyo.