Complex sentence: He retired in 2004 at 29, because of back injury, and has since taken up rowing winning his age group and coming third overall in 2008 and beating international rowers to win overall in 2009 at the British Indoor Rowing Championships.
Split into simpler sentences: He retired in 2004 at 29, because of back injury, and has since taken up rowing. He has had notable success in indoor rowing, winning his age group and coming third overall in 2008, and beating international rowers to win overall in 2009 at the British Indoor Rowing Championships.