Richard Donovan is on course to complete his final run in Sydney and smash the five-day barrier.
Mr Donovan, a 45-year-old father-of-one, began his epic journey in the extremes of Antarctica, running the first 26-mile leg in -20ºc at the Russian Novo science base last Wednesday.
His current record stands at five days, 10 hours and eight minutes and he has been aiming to knock half a day off that.
If he finishes as planned, his round-the-world marathon will have seen him run in Antarctica; Cape Town, South Africa; Sao Paulo, Brazil; Orlando, Florida, the US; London; Hong Kong; and Sydney.
With a deadline of 7pm today, Mr Donovan will have run 183 miles and flown 27,055 miles.
Running times are independently verified at each location.
He is doing the endurance challenge on a shoestring budget out of his own pocket to raise awareness of the humanitarian crisis in the Horn of Africa.
Mr Donovan, who was inspired to attempt the challenge in 2009 after the failed bid by British explorer Sir Ranulph Fiennes, said he was not on any special diet, was flying economy on his own and was bringing whatever he could fit in one bag.
Before setting off, the Galway man — an experienced marathon runner at both poles — said sleep deprivation and temperature fluctuations would be his biggest challenge.
Money raised through online donations at worldmarathonchallenge.com will go to Irish aid agency Goal, one of the few charities working in the Horn of Africa.