Kimwei won a team silver medal as a junior at the 2004 IAAF World Cross Country Championships. She holds the world junior bests for the 15K and 20K distances, with times of 47:39 minutes and 65:04 minutes, respectively. Both these marks were achieved at the 2006 Kobe Women's Half Marathon, which she won. Kimwei was based in Japan for much of her teenage life and was highly successful there. She won the Matsue 10K and Sanyo Women's Half Marathon in both 2005 and 2007. She also had top three finishes at the Nobeoka Golden Games, Shizuoka International, Hyogo Relays and Sapporo Half Marathon. She was the 2007 Japanese Corporate champion over 10K.