Most people would count themselves lucky to live to a hundred. Being able to run a marathon at that age would be close to a miracle.
As part of Al Jazeera's Still Going Strong series, Tania Page went to meet Fauja Singh, who is racing past that milestone birthday.
Singh was born in India and started running marathons after moving to Britain, in his eighties.
His last was in Toronto, Canada, where - in eight hours, 25 minutes and 16 seconds - he became the world's oldest marathon runner.
Others may struggle to find time to exercise, but Singh, still fit and healthy, is determined to carry on.
Singh attributes his longevity to ginger curry, cups of tea and "being happy".
He admits to getting weaker, but is reluctant to say the London marathon will be his last, because what keeps him going is that he always has a goal to strive for.