"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?"