Flight Haneda Airport departs from Tokyo Haneda Airport at 20:00 and arrives at Seoul Incheon International Airport at 22:30.
The flighttime is about 2h 30m. The distance travelled ("as the bird flies"): 1487 Km. So the average speed is roughly 595 Km.
The best day to arrive at Tokyo is Friday and it would be best to avoid Tuesday, as prices are on average 118.2% higher than on Friday. (Average prices, based on 1485652 datapoints.)
We have a lot more statistics for flights NH 867 and between Tokyo and Seoul.