It is the world’s oldest annual marathon, but what happened in Boston on April 15, 2013 changed the historic event forever. Three people were killed and more than 260 were injured after a pair of bombs detonated near the finish line. Among the most severely wounded that day was Adrianne Haslet-Davis, a professional ballroom dancer who lost a part of her left leg. Her three-year recovery has been painful and difficult, but resilience and determination have her standing, even dancing, again. Norah O'Donnell reports.