The full distance is 42.195km but the other metric on James Walton's mind as he tackles his 22nd consecutive London Marathon tomorrow morning is one millimetre.
The Briton, Deloitte's South-east Asia sports business group leader and travel, hospitality and services sector leader, is running to raise $25,000 for social service organisation AWWA but is mindful of a serious knee injury he is carrying. He has raised $15,000 so far.
Regardless, Walton, who has been in Singapore since 2010, will numb himself to the sharp pain in his right knee to complete the race.
"Living in the UK, the London Marathon is a huge thing, on a par with the FA Cup final (football) and the Grand National (horse racing). There are over a million spectators and 40,000 runners," he said. "It has the most elite field of Olympic and world champions. It has a certain aura and stands alone among marathons."
Indeed, this year's London Marathon will feature Kenya's Olympic champions Eliud Kipchoge and Mary Keitany, Britain's Mo Farah... and foul-mouthed celebrity chef Gordon Ramsay.
Walton first took part in the London Marathon when he was 19 and it just became a fixture in his calendar. "It became a streak that I want to keep alive," he said, no matter what part of the world he is in.
But the 40-year old will need plenty of fortitude to cope with the strains of both the race and the torn cartilage in his knee, which shows up as three holes on an MRI.
"I had a bad run (of injuries) in the last 18 months," he recalled.
"I was told to forget about the London Marathon. That really hit me like a thunderbolt."
Still, Walton, who was also diagnosed with osteoporosis at 17, refused to surrender. Seeking a second opinion, he was told he could still participate in his annual ritual and go under the knife later.
The biggest of the three holes measures 9mm now. But, if the tear extends by another millimetre, it would call for major surgery and at least six months of recovery time.
"The furthest I ran was 15km during training," he said. "I have enough stamina for about 24km, anything further than that will be on a wing and prayer."
And, if he can recover in time after the London Marathon, he has already pencilled in The Straits Times Run on Sept 23 as his comeback. He hopes to run in the 10km category not just to confirm that his knee is trouble-free but also to pound the pavement for a good cause for AWWA once more.
He said: "I have since raised money for guide dogs for the blind and In My Shoes (a movement he founded with national hurdler Dipna Lim-Prasad to collect and redistribute sports shoes to the needy). At Deloitte, we are making a big push for AWWA in autism.
"My mother had polio and had been paralysed from waist down since she was eight. That gave me a very good focal point on why I should run."
•Readers who wish to back Walton can donate at www.giving.sg/ campaigns/jameslondon2018