I just found my 10 page document of Andy Milroy's article "The Greatest 100k" again.
I couple of remarks.
When Don Ritchie was running his world record on track there were no accurately measured 100k races yet.
That came in the mid 80ies spearheaded by Harry Arndt. His 100k race in Hanau Rodenbach was the gold standard for 100k races. A 10k loop accurately measured. At that time most 100k races also the one loop races got remeasured and the courses lengthened to real 100k.
That also included Western States 100, which had his first real 100 miles in 1985 or 86.
Tracks in the 70ies where not the high performance tracks of today.
So it was a hell of an accomplishment in 1978 and that the record stood for 40 years and only got improved minimal shows who good that performance was.
And here you can read it yourself
https://ultrarunninghistory.com/the-greatest-100-km/