MarathonManUK runs his 384th marathon and in his record time too!
There were all the usual crazy costumes, and even a couple getting married en route, but amongst the tens of thousands of people pounding London's streets today, there was one man with a particularly astonishing story.
Last year Rob Young had never run a single marathon - now he has 384 under his belt.
Starting in London's parks and commons, he's run more than a marathon a day for the last 12 months - crushing the world record for the furthest run in a single year and earning the nickname "The Marathon Man".
He'd been sitting at home, watching the London Marathon last year, when his fiance dared him to try one too. The next day, he got up and ran one round Richmond Park. And from there, it snowballed.
Doctors are so astonished at his endurance, they've done special tests to find out how he does it. The Assistant Medical Director for the London Marathon, Dr Courtney Kipps, says he's never seen another athlete like him.