text: Dean Karnazes finishing his historic Run Across America in 2011. His message to Greeks: 'You are better than you think and you can go further than you think.' Photograph: Mike Disciullo/Bauer-Griffin/Eroteme.co.uk
  
  Dean Karnazes is not a man to shy away from challenges: like Pheidippides, the original marathoner who delivered the news of the Greek army's victory over invading Persian forces, he doesn't give up.
  
  Seven years ago, he ran 50 marathons in 50 consecutive days from one coast of the United States to the other. Before that he spent three days and three nights running 350 miles – the last night "sleep running" as he went. He's crossed all four of the world's great deserts, including Death Valley, the hottest place on Earth. And he's run through arctic temperatures to get to the south pole, the coldest place on Earth. In 2015, he hopes to traverse the world by clocking up marathons throughout the year in 204 countries.
  
  But now the Californian has his sights on crisis-hit Greece, the land of his forebears, where he arrived this week on a mission of endurance and hope.
  
  "I come with a message," he said on the first day of his first ever visit to the Greek capital.
  
  "And that is that you are better than you think and you can go further than you think. The only limits you have are you own preconceived notions of what you can't, and can do."
  
  On Friday, the 51-year-old ultra-marathon runner – described by some as the fittest man on the planet – takes that message to the Peloponnese, one of the poorest regions in a nation that now holds the unenviable position of being the unhappiest in Europe.
  
  There he will run a marathon from his ancestral village, high in the mountains of Arcadia, to the Messinian plain in what will be a triple-pronged effort to promote sports tourism to the debt-stricken country, highlight the perils of child obesity – a growing problem in Greece – and raise awareness of the benefits of the Mediterranean diet to which he subscribes religiously.
  
  "Hardship and adversity make you stronger," he insisted with his parents, both second-generation Greek-Americans, looking on. "Change is never easy but the Greeks have clearly realised that they couldn't keep going down the same path. As painful as it has been, there has been somewhat of a cleansing of the economy and with that has come hope."
  
  Although fascinated from a young age by endurance and discipline – the very virtues Greece now needs most – the athlete readily acknowledges that he also has "certain gifts".
  
  Almost uniquely, scientists have found that he does not accumulate lactic acid, which enables him to run seemingly forever without enduring muscle pain or cramps.
  
  "My biomechanics are unique," said Karnazes while enthusing about one of his most recent "jaunts", a 75-mile, 14-hour run from San Francisco to Healdsburg, the wine capital of northern California, to attend a friend's party. "For a long-distance runner it's the perfect body alignment."
  
  But the Greek-American, who will be met at the finishing line on Sunday by Antonis Samaras, the prime minister of Greece, also admits that it took time and effort "to manifest" his gifts. "I wasn't always the perfect athlete … I used to eat a lot of junk food but have changed over the course of two decades," said the former business executive. "I now eat only healthy, natural foods."
  
  Karnazes, who turned to running on his 30th birthday – challenging himself after a night of heavy drinking to run 30 miles – hopes his own tale of personal transformation will inspire Greeks.
  
  "A lot of younger Greeks often reach out to me for advice and I am seeing a lot of optimism among them," he said, adding that he hoped Greece would become a prime destination for "active tourism".
  
  "There is a message of hope for people in what I have done. They can change for the better." In Greece, the land of Pheidippides, helping change – like crossing the world – would be his biggest dream yet.
summary: The ultra-marathon expert, described by some as the fittest man on Earth, is running from Arcadia to Messinia to highlight child obesity in Greece