Flight Haneda Airport departs from Tokyo Haneda Airport at 02:00 and arrives at Seoul Incheon International Airport at 04:35.
The flighttime is about 2h 35m. The distance travelled ("as the bird flies"): 1487 Km. So the average speed is roughly 576 Km.
The best day to arrive at Tokyo is Sunday and it would be best to avoid Thursday, as prices are on average 696.15% higher than on Sunday. (Average prices, based on 1365708 datapoints.)
We have a lot more statistics for flights JL 5257 and between Tokyo and Seoul.