Flight Incheon International Airport departs from Seoul Incheon International Airport at 17:40 and arrives at Tokyo Haneda Airport at 20:05.
The flighttime is about 2h 25m. The distance travelled ("as the bird flies"): 1487 Km. So the average speed is roughly 615 Km.
The best day to arrive at Seoul is Friday and it would be best to avoid Wednesday, as prices are on average 148.5% higher than on Friday. (Average prices, based on 2561430 datapoints.)
We have a lot more statistics for flights ET 672 and between Seoul and Tokyo.