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Athlete Johanna Kariankei stretches on the dirt track in Iten, the Kenyan town perched 8,000ft above sea level on the edge of the Rift Valley escarpment that has become the Mecca of high-altitude endurance training for the world's best long-distance runners. Few of these amateurs have access to professional facilities, yet the town hosts many of the fastest men and women over marathon, half-marathon and 10,000m distances.