When Katie O’Donnell got engaged in November 2015, she never imagined she would be married a mere three weeks later within the walls of Massachusetts General Hospital, after a shocking leukemia diagnosis that required immediate treatment.The Derry, N.H., resident is just one in a long list of cancer patients who have benefited from Caring for a Cure — a charity that makes life in the hospital a little brighter, started by MGH nurses who are gearing up to run the Boston Marathon on Monday.