She came first in the 5000 metres at the 2004 Japanese Athletics Championships, competing as a guest. Following this she was the runner-up in the Kenyan Olympic trials over 10,000 m. Her first major international appearance came at the 2004 Olympics in Athens, where she finished ninth in the 10,000 metres event. She ran over that distance at the Hyogo Relays meet and had three straight wins from 2005 to 2007. She finished tenth in 5000 metres at the 2005 World Athletics Final. Wangui made her international debut on grass that year at the 2005 IAAF World Cross Country Championships, where her fifth-place finish in the short race made the Kenyan team a close second to Ethiopia in the team rankings.