Mr Butter, of Dorset, said: "It's been the most incredible experience, I've seen things you wouldn't even imagine as I've run across the world.
"It took two years to plan which was really daunting, sorting out all the logistics and organising the flights and everything.
"Over the past 18 months, I've been through 10 passports and something like 120 Visas, I've lived in airports and hotel rooms, it's just been a total whirlwind.
"It's been such an eyeopening adventure. "It's gone so fast, yet at the same time feels like I've been doing it forever, it's a weird reality and I can't believe I'm at the finish line."
Mr Butter has always been a keen runner, taking part in marathons across the globe, but his world changed in 2016 when he met fellow runner Kevin Webber during the Marathon Des Sables in Morocco.
After being diagnosed with prostate cancer in 2014 and told he had two years to live, Kevin had set off running marathons across the world to raise money for Prostate Cancer UK.
Nick said: "I couldn't believe that this guy, who always seemed so happy and full of life, had just told me he had terminal cancer.
"I was shocked, and when Kevin told me 'Don't wait for the diagnosis' it really struck a chord with me and I just knew I had to do something to raise money for the charity."
To donate visit Mr Butter's Just Giving page at /www.justgiving.com/ fundraising/runningtheworld.
A former banker has become the first person to run the length of a marathon in every country in the world.
Nick Butter, 30, set a world record after running 26.2 miles in 196 countries recognised by the United Nations. He completed his final journey in Athens, Greece, on Sunday.
Mr Butter quit his banking job to embark on his solo expedition in January last year after being inspired by a friend with terminal cancer.
He has since covered 5,130 miles in organised events across seven continents, overcoming the extreme heat of the Sahara Desert and bitter cold of Antarctica.
During the 675-day adventure he has been hit by a car, bitten by a dog, broken his elbow and even been shot at. He hopes to raise a total of £250,000 for Prostate Cancer UK.