Flight Gimpo International Airport departs from Seoul Gimpo International Airport at 19:20 and arrives at Tokyo Haneda Airport at 21:30.
The flighttime is about 2h 10m. The distance travelled ("as the bird flies"): 1450 Km. So the average speed is roughly 669 Km.
The best day to arrive at Seoul is Wednesday and it would be best to avoid Sunday, as prices are on average 10.72% higher than on Wednesday. (Average prices, based on 1195940 datapoints.)
We have a lot more statistics for flights JL 94 and between Seoul and Tokyo.