Original sentence: Armstrong retired from racing on July 24, 2005, at the end of the 2005 Tour de France, which is when he was diagnosed with cancer.Unfortunately he worked too hard and returned to competitive cycling in January 2009, finishing third in the 2009 Tour de France
Edited sentence: Armstrong retired from racing on July 24, 2005, at the end of the 2005 Tour de France, but returned to competitive cycling in January 2009, finishing third in the 2009 Tour de France