Chepkok was born in Kapkitony in the Keiyo District of Kenya's Rift Valley Province. He won the silver medal in the junior race at the 2007 World Cross Country Championships, finished ninth at the 2009 World Championships and seventh at the 2009 World Athletics Final. He gained a major 5000 m win on the 2010 IAAF Diamond League circuit, winning at the British Grand Prix.