Doing a quick google search, I came across an article on the Finding Dulcinea website that places the Ironman Triathlon among the top five hardest races in the world.  There is certainly no doubt that it is a grueling event, one that presses the limits of the body and the mind.  At one point the marathon was considered the ultimate endurance feat, this event punishes the athlete through hours of exercise before finishing with a marathon.

In 1977 some athletes were debating who was more fit; runners, swimmers, or perhaps cyclists. This debate evolved into a challenge that took place in Hawaii the following year.  15 men set out to complete an endurance event that far surpassed anything else that was going on at the time.  The race was to start with a 2.4 mile swim, followed immediately by a 112 mile bike race and then finished with a full 26.2 mile marathon.  Out of the 15, 12 managed to complete the event.  The winning time for that race was 11 hours 48 minutes.  Not bad considering most of the athletes had not done much, if any training outside of their primary sport.  That event launched the beginning of what is today the Ironman World Championships and the Ironman and ½ Ironman race series.

Today the Ironman sanctions 25 full Ironman races around the world, 8 of which are held in the United States.  The 70.3, or ½ Ironman series, has grown in popularity with a total of 40 races worldwide, 20 of which are held in the United States.  All of these events are considered qualifying events for the world championships in Hawaii.  Hawaii Ironman allows 1,800 participants.  Roughly 175 of them are considered professionals, and 200 of them are chosen from a lottery.  I believe there are a number of slots available to charity organizations and the rest are left open for qualified age groupers.  For each of the sanctioned Ironman events there is a certain number of slots available to the top finishers in each age group, varying depending on the age group and the event.  Unlike qualification for the Boston Marathon, qualifying is dependent on your age group placing and not on your finishing time. 

Since the Ironman has been around for over 30 years now, the training is down to a science.  There are many professionals who make a living racing in these events and are able to devote as much as 8 hours a day to their training.  Over time they have conditioned their bodies to not only put up with this kind of punishment, but do it far faster then I am sure anyone finishing the race in 1978 thought possible.  In order to win the race today you must finish around the 8 hour mark.  That 8 hours is typically broken down into roughly a 50-55 minute swim, 4:30-5 hour bike ride, and 2:45 marathon.  All of those times are pretty impressive and these guys have managed to do them consecutively with no break, wow.