Anguish is a visitor at the start line too. When the gun is fired an excited fear hits the road of runners. For newcomers its the promise of unfamiliar territory. The best trainers will tell you that while the lasting benefits of a marathon come in the training, you should never train the full 26 miles (about 42 kilometres). You leave the last six miles "unexplored".

That's because there's a race within a race. At the 20-mile mark muscles can run out glycogen, causing intense pain as the body searches for something else to burn. It's called "hitting the wall". Other human processes can break down as well.

There's no way I'd attempt a marathon. I've been a wheezy asthmatic, fighting for air, since small. But I admire those who do.

My 85-year old father-in-law, Ross, who didn't run his first marathon until he was 50, admits: "It's an insult to the body."

He says runners "are almost unconscious; in a zone of their own".

Ross wouldn't know, but it must be like childbirth. The pain, the zone. Over time one's mind strangely distills the memory into something beautiful.

In the final few paces runners often say, "never again". Forgetting must be the only thing that explains why Ross completed more than 60 marathons, including the famous Boston event, as well as ultra-marathons and totally unfathomable 24-hour runs.

In running retirement he is grateful for the encouragement he got while "in the zone" – a familiar face, the kind voice of a stranger "that lifts the spirit".

Like hot millet bags or soothing words at a birth centre, runners latch on to whatever support is offered (and thank you to the extraordinary natural-birth advocate Sheila Kitzinger, who died this week and told me and millions of other mothers-to-be that we were doing well and should trust our bodies).

In some ways marathons are the opposite of natural. They are acts of will and cajoling in which runners make their bodies do what's required regardless of how ill-suited they are. Marathon runners do not need to be gifted, but they do need to be determined.

They embody the spirit summoned by John F. Kennedy who told Americans in 1962 that they would go to the moon and do a bunch of other things by the end of the decade "not because they are easy, but because they are hard".

Rudyard Kipling writes on the mystery of "will":

If you can force your heart and nerve and sinew, To serve your turn long after they are gone, And so hold on when there is nothing in you, Except the Will which says to them: 'Hold on!'

Richard Flanagan examines the phenomenon in his stunning Man Booker Prize novel The Narrow Road to the Deep North. He describes the will to live, to endure cholera, the endless mud, beatings and hunger for heaven knows what reason on The Line – the Thai-Burma railway. Love of Empire, mates or a fiance back home just doesn't explain it. Flanagan offers mystery over definition.

The marathon was named after a Greek city and the legend of a messenger who ran from the battlefield of Marathon to Athens (perhaps 20 miles) to announce that the Persians had been defeated. The race was standardised at 26 miles and 385 yards in 1921. We know it as 42.195 kilometres. They are now all over the world, even in Antarctica. In Boston (the scene of the 2013 bombing) so many attend from all over the world that distance markers use both metric and imperial measures. In Canberra ours begins near the National Library, reminiscent of a Greek temple.

Unlike other Olympic events, spectators can line the road for free to watch. It's always the last event in the program, the human-sized one, the most democratic. Even if we're shabby at it, we know how to run.

"I was no good at any sport at all," says Ross. "When you find you're not athletic or well co-ordinated, you try slow, long-distance running."

Marathons are a great leveller. In Canberra, diplomats run alongside office workers, all suffering "in good company".

"It's hours of continuous activity, not like other sports where you stop and start and your heart rate goes back to normal."

Ross says his most enjoyable marathons have been when he helped somebody who was struggling in the last stages.

"I reckon I walked across the line four times with somebody that I didn't know, staying with them," he says. "I remember the last person. We talked about her family. You try and take them out of themselves, exchange words. When we came to the finish I held her hand up. Her family saw her and immediately pressed into her arms. I never saw her again."

Toni Hassan is a Canberra writer. Twitter: @ToniHassan.