Despite the pain and the hours of preparation, such endurance events are strangely addictive.
Briton Mark Kleanthous has competed in 34 Ironmans -- including the most famous of them all in the blistering heat and brutal winds of Hawaii -- two double Ironmans and one Triple Ironman.
Since the mid-eighties, the 51 year-old has finished over 450 triathlons, making him almost certainly the record holder in that respect, not to mention the small matter of 74 mere marathons..
"The longer the event, the more the mind takes over, in a marathon it is probably only 10% mental in a Triple Ironman, with sleep deprivation, it must be at least 40% mental strength to continue," Kleanthous, who now coaches and mentors athletes who take on these challenges, told CNN.
Kleanthous took just under 46 hours of continuous action to complete his solitary Triple Ironman attempt.
"After swimming, cycling and running a total distance of 521 miles I had no facial hair growth for 10 days," he said. "I think it was my body's way of saying I am shutting down."
But there can be a darker side to this type of challenge.
"I have known ultra endurance athletes to commit suicide within months of finishing sleep deprivation events," said Kleanthous.
"In events lasting days with no sleep for one or two hours per day athletes have hallucinated and believed they have seen friends or family cheering them by the side of the roads.