Flight Haneda Airport departs from Tokyo Haneda Airport at 19:40 and arrives at Seoul Gimpo International Airport at 22:00.
The flighttime is about 2h 20m. The distance travelled ("as the bird flies"): 1450 Km. So the average speed is roughly 621 Km.
The best day to arrive at Tokyo is Monday and it would be best to avoid Friday, as prices are on average 206.45% higher than on Monday. (Average prices, based on 387085 datapoints.)
We have a lot more statistics for flights JL 95 and between Tokyo and Seoul.