If God invented marathons to keep people from doing anything more stupid, triathlon must have taken Him completely by surprise.
— P. Z. Pearce
So what is this Ironman thing? To give a short summary, it is probably the hardest one-day endurance event in existence. An Ironman is made up of a 3.9 km swim, a 180 km bike ride and a marathon in the end. All done, back to back, without interruptions. It is a grueling and absolutely awesome sport. In the words of someone who has reached the heights of this madness:
Triathlon is work that can leave you crumpled in a heap, puking by the roadside. It's the physical brutality of climbing Mount Everest without the great view from the top of the world. What kind of person keeps coming back for more of that?
—Chris "Macca" McCormack, 2x Ironman World Champion
The more relevant question Macca doesn't address in his comment is, who the hell wakes up one morning and thinks it's a good idea to run a marathon after swimming the equivalent of 78 Olympic lanes and cycling a Tour de France stage?!
To answer this question let's look back to the 70s. In September of 1974 the San Diego Track Club held the first ever triathlon competition composed of a 10 km run, 8 km cycle and a 500-meter swim. The sport would take up a lot of traction gaining Olympic status by 1989 with the inaugural men and women triathlon held at the 2000 Sydney Olympics.
Also taking part in the 1974 and 1975 San Diego Triathlons was a certain Navy commander John Collins and his wife Judy. In 1977, on the island of Hawaii, members of the Mid-Pacific Road Runners and the Waikiki Swim Club were debating who the fittest athlete was. Runners or swimmers? Or maybe cyclists as a certain Eddy Merckx was storming up the roads of Europe with an enormous endurance. To settle the dispute Collins proposed a race that was made up of three already existing competitions on the island: The 3.86 kilometer Waikiki Roughwater Swim, the 185-kilometer Around-Oahu Bike Race and the 42.195 kilometer Honolulu Marathon.
This was the birth of the inaugural Ironman held on February 18, 1978. The 15-men race would be won by Gordon Haller, a US Navy member, in 11 hours 46 minutes and 58 seconds. With his finishing time, Haller was 3 hours, 43 minutes and 2 seconds slower than the current Ironman World Champion record run by Craig Alexander in 2011.
In 1981 Collins had passed the direction of the race on to Valerie Silk who moved the race to Big Island and the date from February to October, resulting in two events in 1982. Without big marketing efforts, the race had gained hundreds of participants among them Julie Moss, a US college student. Moss was comfortably leading the women's race when 2 miles before the finish line dehydration and fatigue overtook here and she was passed by Kathleen McCartney. Moss, regardless of here status did not give up and crawled over the finish line. A scene broadcasted all around the globe that formed the myth of the Ironman where mere finishing is one of the biggest achievements.
After the spectacular finish of Moss in 1982, the world of triathlon exploded both in the United States as well as overseas. To regulate the storm of people that wanted to start in the Ironman qualification races were introduced in 1983 and the Ironman Hawaii was rebranded to become the Ironman World Championship. 1983 also saw the rise of future triathlon legend Dave Scott who won for the first of 6 times.
With the growing success of triathlon, more and more races were added to the qualification including races in Europe.
In 1990 sold the Ironman brand to James Pitzer Gills who turned the cooperation into World Triathlon Corporation(WTC) that is organizing the Ironman branded races today. With this expansion came new races all around the globe as well as an expanded merchandise and marketing concept. The M-dot, the official Ironman logo, often inked onto the calves of finishers, was put onto training sleeves, t-shirts, watches and so much more. The WTC also introduced a new race series the Ironman 70.3 or half ironman. The name stems from the 140.6 miles that compose a full Ironman race.
With the success and expansion of the WTC more and more sponsors poured into the sport. They either try to sell their tri-related products to competitors or come in for the lifestyle and extreme sports style that is attached. This has caused a number of high profile Olympic distance triathletes to change over into Ironman triathlon. The most notable here probably being Jan Frodeno who, in winning the Ironman World Championship in 2015, became the first athlete to win both the Ironman World Championship and an Olympic gold medal(Beijing, 2008).