Flight Gimpo International Airport departs from Seoul Gimpo International Airport at 12:05 and arrives at Tokyo Haneda Airport at 14:15.
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 month to arrive at Seoul is March and the most expensive month to fly is July. (Average prices, based on 857 datapoints.)
We have a lot more statistics for flights JL 5707 and between Seoul and Tokyo.