Tim Durbin is the only American competing in the World Marathon Challenge that started last Saturday and ends in Sydney on Friday local time.
So far, the 31-year-old has run six marathons, each 42.195 kilometres, through snow, rain and jetlag in Antarctica, Chile, Miami, Madrid, Morocco and Dubai.
He said in an email that the Morocco marathon was the toughest. It started less than seven hours after he completed the race in Spain.
He says at one point this week he had slept just nine hours in 48 hours and ran three marathons. He says he's slept mostly on planes.
The other competitors are from Brazil, India, Great Britain, Hong Kong, France, Finland and Australia.