Flight Haneda Airport departs from Tokyo Haneda Airport at 09:00 and arrives at Seoul Incheon International Airport at 11:20.
The flighttime is about 2h 20m. The distance travelled ("as the bird flies"): 1487 Km. So the average speed is roughly 637 Km.
The best day to arrive at Tokyo is Monday and it would be best to avoid Thursday, as prices are on average 169.8% higher than on Monday. (Average prices, based on 1331696 datapoints.)
We have a lot more statistics for flights NH 6983 and between Tokyo and Seoul.