I think it is not correct. The main track can only sustain 120 kph which is 75 mph. Due to number of speed restrictions the ave speed comes not more that 45-50 mph. According to your estimate 500 kms is 312 miles when divided by 6 hours will give you 312/6=52 mph. One kilometer is 5/8 of a mile. Correct your estimates/mathematics.:)
track condition depends on where you are, from karachi to hyderabad it took 2 hours and speed around 60-80km, but hyderabad onwards it was blazing fast my estimate above 100 in range of 110-120.
yo cannot calculate 312/6 not considering stopovers, i.e atleast 40 mins including slow downs around stations and stops.
so do the match again (312/5) x (8/5) around 92km/h .
and if you do the average based on longer distance to lahore it would surely come well above 110-20 .