I found this in the Ironman website and I think it resumes a lot of what this sport is all about!
"In 1978, U.S. Naval Commander John Collins and his wife Judy issued a challenge. They proposed combining Oahu’s 2.4 mile swim race, 112 mile bike race, and 26.2 mile marathon. On February 18, 1978 fifteen competitors, including Collins, traveled to Waikiki to take on the first-ever IRONMAN challenge.
Swim 2.4 miles, bike 112 miles, run 26.2 miles- BRAG FOR THE REST OF YOUR LIFE. "