The older sister of 10,000 metres World Champion Sally Barsosio, Florence began her career as a track and cross country runner. At the 1995 World Championships in Athletics she finished 13th in the women's 5000 metres final. She was the 1996 winner of the Cross della Vallagarina, Montefortiana Turà and Giro di Castelbuono competitions. She also finished in 13th place at the 1997 IAAF World Cross Country Championships in Turin. However, it was not until she began focusing on road running in the early 2000s that she found greater success.