Flight Haneda Airport departs from Tokyo Haneda Airport at 20:05 and arrives at Seoul Incheon International Airport at 22:20.
The flighttime is about 2h 15m. The distance travelled ("as the bird flies"): 1487 Km. So the average speed is roughly 661 Km.
The best day to arrive at Tokyo is Thursday and it would be best to avoid Saturday, as prices are on average 86.15% higher than on Thursday. (Average prices, based on 1239587 datapoints.)
We have a lot more statistics for flights NH 867 and between Tokyo and Seoul.