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Triathlon is the sport (three sports, typically swim - bike - run, though Winterlude in Ottawa does Ice Skate, Cross Country Ski, Run).
Ironman is the distance. 2.4 Mile (3.8K) swim, 112 mile (180.2K) bike, and a marathon 26.2 mile run (42.2K).
What is the origin of these distances?
The distances were originally copied from three existing races. There was already a 2.4 mile swim race, and a bike race with a similar distance, and of course the marathon. The thought was "what if we did all three?" More here: http://en.wikipedia.org/wiki/Ironman_Triathlon
Hopefully the marathon distance should be obvious: it's a standard distance in athletics, although its actual length (42.2km, 26.2 miles) is irregular. The distance originally varied, in the neighborhood of 24-25 miles, but became standardized after the 1908 London Olympics.
