Flight Haneda Airport departs from Tokyo Haneda Airport at 08:20 and arrives at Seoul Incheon International Airport at 10:55.
The flighttime is about 2h 35m. The distance travelled ("as the bird flies"): 1487 Km. So the average speed is roughly 576 Km.
The best day to arrive at Tokyo is Saturday and it would be best to avoid Wednesday, as prices are on average 40.79% higher than on Saturday. (Average prices, based on 1488238 datapoints.)
We have a lot more statistics for flights KE 5708 and between Tokyo and Seoul.