That's strange -- a runner from Great Britain just won the Olympic gold medal in men's 10,000 metres. I thought it was Ethiopians who ruled at running long distances... Ah wait, his name is Mohamed Farah and he's Somali.
What explains the dominance of long-distance running by Ethiopians, Kenyans, and other related people in that corner of the world? Very simply, it is their adaptation (through natural selection) to a way of life called nomadic pastoralism. They gain their subsistence by herding livestock rather than hunting and gathering, tending small-scale gardens, or cultivating larger fields of crops. The nomadic groups of pastoralists tend to live in harsher climates with little arable land, so they wander over great distances in search of scarce pastures for their herds.
Having to endure a continual trek across such vast distances must have selected for the ideal physiology of long-distance runners. You don't really need too much explosive strength like you do in sports that involve sprinting, jumping, and throwing, or those that involve hitting projectiles with shock weapons. You just need to be able to move yourself on foot for a very long stretch of land without getting too tired.
The nomadic form of herding livestock is more common in the East African highlands, across the Sahel into West Africa (trekking through the Sahara Desert), and in Central Asia, with patches across the Middle East. There are also spill-overs of pastoralist groups into neighboring countries that are generally not pastoralists. And not surprisingly, the few excellent endurance runners from these non-pastoralist countries are actually pastoralist minorities. For example, one of the top 20 times in the men's 10K is held by Boniface Toroitich Kiprop, technically from Uganda, whose population are mostly gardeners, and a country with limited success in endurance sports. However, the district that he's from is right on the Kenyan border and is mostly populated by cattle-herders.
So far the Central Asians haven't come to rival the pastoralist Africans in the 10K, marathon, etc., although I'm not sure how interested they are in Olympic competition. Alternatively, the Africans may have an extra advantage from being genetically adapted to high altitude. (That can't be their primary strength, though, since no other people from high-altitude regions dominate long-distance running.)
Apparently the nomadic pastoralist way of life has not only selected for superior endurance among the human herders, but also among their horses. In the world of equestrian sports, endurance riding tends to make use of various Oriental horse breeds -- "Oriental" in the older sense of the Near and Middle East -- such as the Arabian. These breeds are famed for their endurance over long distances, unlike those that have been bred to be beasts of burden, who will only have to traipse around the farm of a settled agriculturalist.
There is another form of pastoralism, called transhumance, where the herders don't travel such great distances to unpredictable locations, where instead they have a more fixed pattern of movement between summer and winter pastures. Generally this means the summer pastures are at higher elevation and winter pastures at lower elevation. The herders have more permanent homes near each place, and their migration is a predictable over time -- being seasonal -- and over space -- going from one known choice location to another.
That would seem to select for a greater level of endurance, but just not at the level of the more nomadic groups that rule at distance running. Also, the setting up, maintenance, and defense of their relatively more permanent settlements would keep the pressure on their ability to use strength, as well as endurance, to make a living.
Transhumance is found across the Near and Middle East, the dairying northwestern part of South Asia, Southern Europe, the Alps, Scandinavia, and the British Isles (particularly the Celtic groups). Again it's mostly where there are mountainous pastures to thrive on in the summer, plus lower-lying pastures for the winter.
What endurance sport do these types of pastoralists dominate? Soccer. It is mostly an endurance sport, but it also features rare but important bursts of speed, not to mention giving the ball an occasional good hard kick. The people who dominate at soccer are the cheese-making countries of Europe, particularly the more southern ones, where once again adaptation to higher altitudes may give them an extra advantage.
The Northeastern region of Europe, where large-scale agriculture has traditionally excluded pastoralism, tends not to do very well in soccer, especially adjusting for population size. For example, Russia has a population of around 140 million, while Spain's is nearly 50 million, so the best soccer players in Russia are a much higher percentile within the country. With so many more people, they should be able to find more of those rare gems, and yet they perform more poorly than the Spanish at soccer.
The Near and Middle Eastern countries seem to do OK at soccer, but not as good as the transhumance pastoralists from Europe. Soccer is a team sport, so regions with lower social cohesion won't produce the greatest teams, even if their individual athletes have the right physiology. Societies with higher levels of pastoralism tend to be more rugged individualists, whereas in Europe pastoralism has always co-existed with settled agriculture as well, where people will put up with their neighbors for collective welfare.
And of course anywhere else in the world that these societies have colonized, also do very well in soccer. Brazil and Argentina are the obvious examples, where they were seeded not only by Southern Europeans, but where they continued their cattle-herding way of life. Other countries like Uruguay, Chile, and Mexico, do well in soccer, but they also tend to have more indigenous mixture, and the gardening societies of the Americas have not selected for endurance. Also, the cowboy culture did not thrive as strongly in these latter countries as in Brazil and Argentina, who are still leading beef producers.
What about the Scandinavians? They aren't so hot at soccer, but they do dominate in cross-country skiing. That can't be because they're more familiar with snow, because on a per capita basis Norway and Sweden leave Russia in their dust. But Norwegians and Swedes have been shaped more by transhumance pastoralism than have the more strictly agriculturalist Russians. Also, you don't find Eskimos or other Arctic peoples excelling at cross-country skiing, and they're more familiar with cold weather and the snow than anyone. Their way of life was traditionally hunting and gathering, which in the African savannah might have involved lots of moving on foot, but in the Arctic has relied heavily on boats (for marine hunting) and sleds pulled by a team of animals (for travel over the land).
I hope this little tour through human biodiversity will encourage the HBD crowd to study and apply what is well known about different types of subsistence -- hunting and gathering, horticulture, agriculture, and pastoralism. Getting a hold of subtler differences within each type would do even better. Too much thinking about group differences -- as rare as that already is -- focuses on continent-level races, lumping together the very different Mongolians and Han Chinese, or societies at different levels of economic and political development, ignoring the profound differences between the more advanced East Asian countries like Japan, South Korea, and China, and the more advanced European countries, because hey, they're all smart, hard-working, and economically globally competitive.
I don't mean to get so snarky, but you do see too much of an East Asian fetish among many who are into HBD, and a basic awareness of how different subsistence types lead to different higher-level social properties would correct this naive fetish.