The most common reason put forward is being born at altitude. Many runners hail from regions above 5,000ft. Certainly, living in the thin air helps. Athletes find themselves stronger, especially when they return to the abundant oxygen at sea level.
THE SCHOOL RUN
Most Kenyans have run since their younger days – up to 40km a day. Former 3,000m world record-holder Daniel Komen says: "Every day I used to milk the cows, run to school, run home for lunch, back to school, home, tend the cows. This is the Kenyan way."
EAT UP THE MILES
Many sports scientists believe that the carbohydrate-rich diet consumed by Kenyans might play its part in their success. Even when competing in the west they prefer to eat their starchy dish of ugali, which is maize flour cooked with water.
RUN FOR YOUR MONEY
The riches gained from track and field are a huge incentive as one big win could turn their lives around. Prize money and endorsements can be substantial – buying a farm for around US$20,000 will set up an athlete for life.
RUN FOR THE HILLS
A landscape of rolling hills is a perfect environment. Warm days followed by cool nights and low humidity provide the ideal blend. The climate means that Kenyans can train all year round – as in Jamaica with its history of sprinters.
Compared with many African nations, malnutrition is rare in Kenya with life expectancy and literacy among the highest. But Kenya is not economically strong and there something of the "hungry fighter" syndrome among those who take up running.
While the training of an elite long-distance athlete may not differ hugely from one country to another, the Kenyan model has some unique aspects to it, combining huge mileage with massive intensity – a training level which not every athlete can cope with.
RUNS IN FAMILY
Almost three-quarters of all of Kenya's athletics medals have come from one tribe, the Kalenjins, who live in the lush Nandi district at the edge of the Great Rift Valley in the Kenyan Highlands. The tribe comprise a little over one 10th of the nation's population.
STEAL A MARCH
An expert on Kenyan running, John Manners, says that while most Kalenjins are farmers, their traditional livelihood came from herding or long-distance cattle rustling. The fastest brought home the stolen livestock and the slowest were caught and killed.
Kenyans seem to produce less ammonia, which is what causes fatigue during exercise, and they rely more on fat for fuel rather than protein – which is probably why they do produce less ammonia. Both have benefits for long-distance running.
Kenyan youngsters tend to be thin with spindly legs, though quite healthy. One study shows Kenyans have less mass for their height, longer legs, shorter torsos and slender limbs, traits that make them more efficient over long distances and able to absorb heat.
RUN FOR YOUR LIFE
Whereas in the West the best athletes often go into football, basketball and other ball games, the Kenyan culture, like that of neighbouring Ethiopia – another highly successful producer of marathon and long-distance athletes – is one of running.
Coaches say motivation seems to drive Kenyan athletes more than most think they can endure. Schoolchildren are encouraged to compete against each other at an early age, with prizes up for grabs, so if they are successful they will want to keep training.
A country so rich in raw athletic resources in men and, more recently, women has attracted many of the world's top coaches, whose expertise helps to refine the technique of home-grown athletes who have now become based in US colleges or in the UK.
Tests show that Kenyan runners can maintain 92 per cent of VO2 max (the maximum capacity of the body to transport and use oxygen during exercise) for extended periods while most other athletes can only manage 87-88 per cent, so they expend less effort.
AN EARLY START
One top coach – who has at least six world champions on his books – says it takes at least 10 years of training to build enough of an endurance base to allow athletes to excel at long-distance running "but by the time a Kenyan is 16 he is already there".
The Kenyans often run to orders as a team, showing a discipline – both on and off the track – not always evident among other leading nations. And if you don't succeed, you go back to cattle farming, while if you do, you can become a wealthy national hero.
STRENGTH IN DEPTH
For every successful Kenyan athlete, there are 10 others busy training in the hope of achieving the same levels of achievement. For them, making it as a runner, even modestly, is their best means of a chance of escape.
Kenyans don't understand the fascination with jogging as a leisure pursuit. It is normal for them to train twice a day, six days a week. Some even four times a day. And they all try to out-do one another in terms of who can train the most or the hardest.
Training is done on soft ground, so their joints are spared the impact of running on concrete which improves their elastic rebound, and they are stable runners. They don't try to build up physical strength through weight-training, leaving their joints more flexible.
TOP 20* (Based on last year’s times)
1 Patrick Makau Musyoki
2 Wilson Kipsang Kiprotich
3 Emmanuel Kipchirchir Mutai
4 Geoffrey Kiprono Mutai
5 Levy Matebo Omari
6 Albert Kiplagat Matebor
7 Wilson Kwambai Chebet
8 Vincent Kipruto
9 Moses Cheruiyot Mosop
10 Martin Lel
11 Jafred Chirchir Kipchumba
12 Laban Korir
13 Erick Ndiema
14 Philip Kimutai Sanga
15 Wesley Korir
16 Nathaniel Kipkosgei
17 Bernard Kiprop Kipyego
18 Robert Kiprono Cheruiyot
19 Benjamin Kolum Kiptoo
20 Peter Cheruiyot Kirui – The Independent on Sunday