In the 2007–08 indoor season Chepkirwok competed at the 2008 World Indoor Championships. He reached the semi-final round, finished fourth there but did not progress to the final. During the summer he performed better. He opened with a second place in Doha, then won the ISTAF Golden League meet in Berlin. He then finished second at the Grand Prix meets in Madrid and Athens, and won the Super Grand Prix meet in Crystal Palace, London. In Madrid he had achieved a personal best time is 1:43.72 minutes, which is also the Ugandan record. Entering the 2008 Olympic Games, then, Chepkirwok was the only middle distance runner in the Ugandan squad. In the 800 metres competition he did not reach the final, however. He rounded off the season with a second place at the 2008 World Athletics Final. In early 2009 it was announced that Chepkirwok was a candidate for the title Ugandan Athlete of the Year. He eventually finished second runner-up for the award, behind Moses Kipsiro and Benjamin Kiplagat.