Can you guess this outdoor event?

Can you guess this famous sporting event from these photos and facts?



Over 25,000 individual participate in this race. It always occurs on a Monday in April, and on a state holiday (Patriot's Day). Participants travel from over 50 states and 60 countries to run this race.



Here is an article describing how this event (and others like it) are trying to become more green and leaving less of an impact on their communities.
http://www.energytribune.com/articles.cfm?aid=1622