A brisk walk is 4 miles per hour and an average walk is 3 miles per hour. If you do the race at a slow walk, 2 miles per hour, you'll do the 100 miles in much less than the allotted 60 hours.
Which means that the elevation and sleep deprivation must really take their toll. Those are the limiting factors, not running speed.
100 miles is 160 km. Tour du Mont Blanc is a 170 km trail around Mont Blanc and surrounding mountains, crossing from France to Italy and then Switzerland before returning to France.
Ultra-marathon runners can do it in 20-45 hours. They carry little but gel packs/etc.
My brother and I hiked it a few years ago, carrying a tent, cooking gear, food, etc. It's a lot of up and down and even if the down doesn't exhaust you, it grinds your knees. Elevation gain is 9,600m (so 31,000 feet). We did it in 9 days, from memory. Could've done it more quickly or walked longer each day but we generally allowed time to select a secretive but flat camping spot (camping wild on the Italian side is somewhat illegal), cook, wash clothes in the river, etc.
The up hill bits definitely slow you down.
Afterwards, we said "Never again!" but I haven't stop thinking about it since and would go back for sure. Highly recommended. Stunning scenery and a great challenge.
actually there are 60000ft of ascent and 60000ft of descent in that race. With steep and or bad condition of descents (check the photos from the links others posted), you need to apply the rule (at least partially) to descents too. Thus we come almost all the way to 90hrs.
Which means that the elevation and sleep deprivation must really take their toll. Those are the limiting factors, not running speed.