- Extreme endurance challenges growing in popularity
- Amateur athletes and celebrities joining professionals on start line
- The 42km marathon run used to be considered the ultimate test
- Many of these endurance events take several days to complete
While obesity rates soar in the developed world and we live an ever more sedentary lifestyle, the flip side to this health time bomb is the paradox that more and more amateur athletes are taking on extreme endurance challenges.
Running a 42km marathon is still considered a huge achievement, but "weekend warriors" have now turned in their droves to Ironman Triathlons.
For the uninitiated, that's a 3.8km swim, 180km cycle race and the marathon to finish.
And the more offbeat the challenge, the more entrants seem to be attracted.
In Telford, England each year thousands of people take part in an eccentric event called the Tough Guy Challenge, which involves a 12km run and assault course in freezing winter conditions.
Just to make it more interesting, the organizer sets fire to parts of the course and puts in barbed wire fences and muddy bogs. Yet they come back year after year to be subjected to this torture.
Worn out just thinking about it?
The Tough Guy Challenge is a relative breeze in comparison to an Ironman race in Norway with arctic temperatures for the swim in a fjord, biking through a mountainous range, then finishing the 42km run at the top of a 1,880m peak.
That's the challenge awaiting competitors in the Norseman, one of 50 events featured in a book, the World's Toughest Endurance Challenges, by Richard Hoad and Paul Moore.
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 75 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.
"One competitor believed he saw his parents and it was only after the race he realized he had hallucinated because his parents were dead."
Despite these warnings, it does not stop the rush to compete in events like the Marathon des Sables, which is also featured in the 50 challenges.
It is a near 200km run in the blazing temperatures and freezing nights of the Sahara Desert -- with a two-year waiting list to compete.
Many competitors are inspired by the exploits of superstar endurance athletes such as four-time Hawaii Ironman world champion Chrissie Wellington of Britain.
"Of all the body parts we train, none is more important than the mind," she told CNN.
"There is an obsession in triathlon with tracking how far and how fast we have gone in our latest session. People think that if their training log is in order, then so must be their preparation.
"But it's when the discomfort strikes that they realize a strong mind is the most powerful weapon of all," added Wellington.
Not all the most famous endurance challenges are multi-sport events, but swimming the English Channel or cycling the near 5,000 km in the Race Across America are still grueling tests.
Extremes of temperature also play a part from the Four Deserts Marathon challenge to the South Pole equivalent at -40C.
High altitude is the enemy in the Yak Attack, a mountain bike race through the Himalayas, while a head for heights is required in the Red Bull X-Alps where competitors paraglide and trek from Salzburg to Monaco across the European mountain range.
Celebrities have also begun to embrace the idea of sporting endurance challenges.
British comedian David Walliams swam the English Channel for charity and topped that by completing the 224km of the River Thames in eight days.
Olympic gold medal winning rower James Cracknell finished 12th in the Marathon des Sables, and then attempted to swim, cycle, run and row between Los Angeles and New York before he was hit by a petrol tanker in Arizona.
It nearly cost him his life, although he has returned to adventure and endurance challenges to raise money for good causes.
Kleanthous is full of admiration for anyone who finishes just one of these events, but then really threw down the guantlet:: "What if someone attempted to do all 50 of the challenges? "