What are you doing on 10-10-10? For 45,000 runners around the country and globe, they can say that they'll be running the 33rd Bank of America Chicago Marathon. For everyone else, especially runners eying a fall marathon to run, they'll just have to be one of the race's one-million-plus spectators. While registration only opened Feb. 1, the Chicago Marathon closed registration only 51 days later. It's a new record for the event, which has only grown and become more popular in the last decade. And in both 2008 and 2009, the race filled in April, a full month later than this year. Not even 10 years ago, the field was smaller and you could still register for the race into the late summer. Now you practically have to know by New Year's that you'll sign up the minute registration opens.