Flight Haneda Airport departs from Tokyo Haneda Airport at 08:40 and arrives at Seoul Gimpo International Airport at 11:15.
The flighttime is about 2h 35m. The distance travelled ("as the bird flies"): 1450 Km. So the average speed is roughly 561 Km.
The best day to arrive at Tokyo is Monday and it would be best to avoid Tuesday, as prices are on average 161.23% higher than on Monday. (Average prices, based on 506522 datapoints.)
We have a lot more statistics for flights NH 6983 and between Tokyo and Seoul.