Flight Haneda Airport departs from Tokyo Haneda Airport at 20:05 and arrives at Seoul Gimpo International Airport at 22:35.
The flighttime is about 2h 30m. The distance travelled ("as the bird flies"): 1450 Km. So the average speed is roughly 580 Km.
The best day to arrive at Tokyo is Saturday and it would be best to avoid Tuesday, as prices are on average 57.72% higher than on Saturday. (Average prices, based on 552583 datapoints.)
We have a lot more statistics for flights NH 6969 and between Tokyo and Seoul.