Flight Gimpo International Airport departs from Seoul Gimpo International Airport at 07:40 and arrives at Tokyo Haneda Airport at 09:50.
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 Friday and it would be best to avoid Saturday, as prices are on average 13.66% higher than on Friday. (Average prices, based on 975330 datapoints.)
We have a lot more statistics for flights NH 862 and between Seoul and Tokyo.