Timbilil started running while at Kemeliet Primary school. She won Kenyan Championships in 10,000 metres in 1999, aged only 16. At the 1999 World Youth Championships in Athletics she won 3000 metres gold medal. She competed also at the 1999 World Championships, but did not finish the 10,000 metres final after seemingly miscalculating the number of laps. She did not start high school until 2000, aged 17, when she joined Kapkenda Secondary School.