 The race is limited to 45 @,@ 000 runners and only runners who finish within 6 ½ hours are officially timed . Those wishing to participate can register after either meeting a time qualifying standard or being selected through a general lottery . Although the race has limited registration , exceptions include elite runners and charity representatives . Increasingly , local ( e.g. , Chicago 's Children 's Memorial Hospital ) , national ( e.g. , American Cancer Society ) and global ( e.g. , Global Business Assist , Team World Vision ) charities and humanitarian organizations encourage sponsored participation in the event as a means of fund raising . 
