It can’t just be elevation, which is what they usually say it is.

https://pubmed.ncbi.nlm.nih.gov/22634972/


Since the 1968 Mexico City Olympics, Kenyan and Ethiopian runners have dominated the middle- and long-distance events in athletics and have exhibited comparable dominance in international cross-country and road-racing competition.

Several factors have been proposed to explain the extraordinary success of the Kenyan and Ethiopian distance runners, including (1) genetic predisposition, (2) development of a high maximal oxygen uptake as a result of extensive walking and running at an early age, (3) relatively high hemoglobin and hematocrit, (4) development of good metabolic "economy/efficiency" based on somatotype and lower limb characteristics, (5) favorable skeletal-muscle-fiber composition and oxidative enzyme profile, (6) traditional Kenyan/Ethiopian diet, (7) living and training at altitude, and (8) motivation to achieve economic success.

Some of these factors have been examined objectively in the laboratory and field, whereas others have been evaluated from an observational perspective. The purpose of this article is to present the current data relative to factors that potentially contribute to the unprecedented success of Kenyan and Ethiopian distance runners, including recent studies that examined potential links between Kenyan and Ethiopian genotype characteristics and elite running performance.

In general, it appears that Kenyan and Ethiopian distance-running success is not based on a unique genetic or physiological characteristic. Rather, it appears to be the result of favorable somatotypical characteristics lending to exceptional biomechanical and metabolic economy/efficiency; chronic exposure to altitude in combination with moderate-volume, high-intensity training (live high + train high), and a strong psychological motivation to succeed athletically for the purpose of economic and social advancement.

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They're winners over so many people
who don't believe in science or evolution.

He said that he felt that he and all of London participated in this record, as he felt their support. I've never run the London Marathon, but I know all of the points on that course well.

The east Africans run from childhood - it's embedded in their culture - run distances to school (Eliud Kipchoge holds one of his arms in a somewhat peculiar position as that's how he held his schoolbooks when he was a boy), and they have the perfect build and physiology to do it. Curiously, according to quite a number of them, most of their countrymen don't continue running as adults, although obviously others do.

The elite runners go out so fast that the pacing bikes have to sprint to stay ahead - I saw this when I was at the Honolulu Marathon when the front pack (pacing bikes/wheelchairs/elite runners) - we hadn't started yet (big crowd) and the front pack had taken the turn onto the main course. I saw the lean cyclists on their racing bikes hauling like crazy up the slight hill to stay ahead of the pack, all in the misty pre-dawn darkness.

Sabastian Sawe, the winner and Yomif Kejelcha, the second place finisher (both of whom broke the world record) were both wearing Adidas supershoes that are 30% lighter and have better energy return than regular running shoes. Women's winner, Tigst Assefa who also broke the world record was wearing the Adidas shoes.

https://therunningchannel.com/the-new-500-shoe-on-the-feet-of-london-marathon-winners/

Centuries of carrying messages over the hilly trails must have led to some form of selection for good runners, even if only social and economic. If successful runners had more children, and/or married relatives of other good runners, or more of their children survived, you'd end up with a statistically significant accumulation of genes which favor the physiology of good runners.

Similar things have happened in ancient societies where one's choice of profession is often dictated by birth -- pursuing your father's profession even may be forced in some cases. Political and social classes, not to mention ethnic associations, may also limit your options. Apparently, if you meet someone unusually tall and strong in Japan, he may be descended from a long line of sawyers -- men who cut trees into boards before sawmills were mechanized -- who were allowed much larger rations of rice than other workers (carbo-loading ! ), by law.