Pushed to the limit: The World's Toughest Endurance Challenges

(CNN) -- 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.

Triple Ironman

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.

Sleep deprivation

"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."

Read: Tips from an Ironman world champion

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.