That's right, and it was achieved with 100mph running. 110mph didn't come in until later. I remember the launch of the Electric Scots. The 5h00 timing was quite a leap forward. I don't know how reliable it was.
On the very first day, the down Royal Scot was 2m36 late arriving.
The 5 hour timing lasted until about 1979, when 8 mins was added. Most of the other services had already had some engineering padding added.
1980 Carlisle was added as a stop.
1981 was the really bad year for the WCML, lots of overdue engineering work saw all journeys extended, the average Glasgow timing became 5h26 down/5h28 up. The Royal Scot now called at Lancaster and took almost 5.5 hours.
1983 things had improved to a standard 5h15 timing, the Royal Scot had 3 stops by now.
1984, the two 110mph runs each way are introduced, Royal Scot down to 5h05 down; 5hr03 up. The other 110mph trains, the Glasgow Executives were 5h05 and 5h15 respectively.
1985, all Glasgow trains timed for 110mph but still nothing achieving 5 hours.
I forget if the 5 hour barrier was broken in 1988, but in 1989 the schedule was 4h45 - the best ever - and finally a timing illustrating the benefit of 110mph running.