In the last year he has run about 1500 miles, gaining new best times in two out of the three marathons he completed, his obsession even taking him to India, where he ran the Auroville marathon through cyclone-damaged temperate forest.
The miles, though, have taken their toll and, early this spring, on a run, he noticed what he thought was a pebble in his shoe. He stopped, tried to shake it out and discovered that what he thought was a pebble was inside his flesh. His foot, as he says, was "crock". "You've got to pay a bit of a price," he says, philosophically. This, however, is a morning in May, and he has yet to hear his doctor's prognosis.
Running is Farquhar's obsession, his meditative space, where he works out his problems, how he defines himself, how he gets himself out of the hole of a hangover. It is also what, this year, his main art work is about: the subject of a spectacular art performance called Speed Of Light that he, and his creative collective NVA, are staging for the Edinburgh International Festival and Cultural Olympiad. It will, over three weeks, take 4000 runners up Arthur's Seat in futuristic light suits to create, for an audience of walkers each night, "an incredible set of shapes and patterns and sense of light moving in landscape, a beautiful thing to watch".
Even the idea for Speed Of Light came out of a run. Farquhar was pounding the paths with the artist Peter McCaughey, a friend he regards as a "great sounding board". They thrashed out, as they ran, ideas about the Olympic spirit and "how to detach it from the corporate and make it into something about how ordinary people might experience something extraordinary". Farquhar has long had a love of Arthur's Seat, having been taken up there as a boy and "taken drugs there as a teenager".
"I love the moment the gorse flowers and you've got a warm day," he recalls. "I've got memories of that smell of coconut. And always, for whatever reason, hills and mountains have loomed large in my imagination. They seem to define something of what I am."
The hills and landscape certainly have long had a presence in Farquhar's work with the arts charity he founded, NVA. Previous shows – The Path, The Storr and Half-Life – have taken their audience out into the Scottish landscape, challenging them, in The Storr, for instance, to hike up by night to the Old Man of Storr as they experienced a sound and light show inspired by the history and geography of the place.
Speed Of Light is less about the "deep history" reflected on in these works and more about the sensory moment. It is, he says, "much more just about movement, energy, light and landscape". Farquhar reflects on the "enlightened commissioning" of Jonathan Mills, director of the Edinburgh International Festival, who backed the work. Mills, he points out, really "got NVA" and was "the first to place us in that Robert Smithson land art tradition. Rather than thinking it was weird sight-responsive theatre, he realised it was in the lineage of people responding to landscape, history and time, and that was crucial."
One of the most innovative aspects of Speed Of Light is the light suit the show's runners will don. The designers have been through six versions of it in the efforts to perfect and it is, Farquhar says, like "science fiction ... very clean, sharp, like something you would see in a film but not necessarily expect to see in the Scottish landscape." Someone, he recalls, once described him as a "futuristic Baden Powell" and this fusion is particularly present in Speed Of Light. Farquhar confesses that he was once in the sea scouts, but was "horribly bullied".
His lifestyle has touches of Green about it, touches of the alternative. He is, he says, "a Buddhist sympathiser" and he also talks enthusiastically about the "spiritual collectivism" he has witnessed in India. His house is distinctive in his street, the only one with a strip of vegetable patch running across the front slope of garden, a straw figure hanging its doorway, a kind of stray wicker man, which it turns out he kept from a Portuguese event he did in 1998. But he is mainstream enough, too, to be working, as he says, as a "state artist".
"I'm funded by the state," he reflects. "In fact, I like saying that I'm a state artist. It seems to me wrong to take the money and then pretend that you are some completely independent individual and can do whatever you want."
For him, however, there is no compromise in Speed Of Light: "This is so obviously close to my heart, I am so inside it." That it is, he says, the "best funded" work he has ever done is perhaps no surprise given how well it fits in with state-approved agendas for improving health in Scotland and Olympiad ideals. Yet Farquhar is not interested in proselytising about exercise. "This is about making something beyond that. Ultimately the 4000 runners and everyone doing it make this sort of ineffable, beautiful thing happen. People will see the hill as they haven't before. So, while of course people can dwell on the social and health outcomes, it's not my driver."
Farquhar, who is now 50, wasn't always a runner. His relationship with running began in 1998 when, after a stressful summer working on the National Day for Britain – in which, commissioned by the foreign office, NVA put on a pagan fire festival and site-specific dance concerts in Portugal – he found himself in Spain, recovering, overweight and in a bad state. He decided to go on a run, and took a quarter mile jog up a hill. The next day he did a half mile, and the next a mile, building up and up until the following year he participated in the Edinburgh Half Marathon, and found that the experience was "the happiest day of my life".
Our muddy path through the Garscube estate takes us close to the river, white rhododendron blooming above us and bluebells shimmering to the right. Farquhar notes, as we turn a bend, that in January, when the snowdrops first come up in triangular bed, this place, for him, is where he senses the turn of winter and sees the first encroaching signs of spring. The year, he says, can be charted here in the flowers that line the path, "snowdrop then crocus then daffodil then tulip then bluebell then wild garlic", in the green that "intensifies and goes from this mustard yellow to something denser".
When people signed up to participate in Speed Of Light, they were asked why they like to run. "So I can eat what I want," was one answer. Farquhar points out that there are there a hundred different ways of running. A "long-term devotee of minimal ambient music", often he puts on a soundtrack and observes the way the music subtly alters the way he sees things as he runs, the way it modulates how he feels. He has days when he focuses on his breath, calming the mind by counting. But the most significant point of the act of going out for a run is, for him, not so much how it is done, but just the following up on the intent of doing it. As he puts it, it's the idea of "I said I would do it, so I will."
That intent is at the heart of his obsession, and also drives his creative productivity. "Whatever in your mind you set, you go out and do," he says. In some ways this last year is a manifestation of the realisation of that intent: Speed Of Light coming to fruition, the 25th anniversary of the Beltane festival which he founded, and the beginnings of a project at St Peter's Seminary in Cardross. But his conversation is also littered with other smaller, personal illustrations.
He recalls, for instance, last Christmas Eve, going out in "the heaviest weather" with his twin girls – them on bikes, him running. They went out to Clydebank along the canal and the wind was so strong they were almost blown backwards off their bikes. "And we stopped," he reflects, "and we had the best donut and cup of tea in the world. And they got it. They did get it."
But this year has also brought something else: an illustration of how will and drive may not be able to conquer everything. Nearly two months on from our initial run, Farquhar has been told that he is unlikely "ever to run another marathon". Without surgery on the damaged area, caused by the misalignment of a toe since birth, he has been advised that he might not even be able to walk in 10 years time.
On his blog he writes, "How ironic, that in the year that I celebrate the joy and fulfillment brought through running, I am well and truly crocked!" But true to form, within days, he is already thinking of ways to circumvent his fate and has bought his first pair of "barefoot shoes" to try barefoot running. There is no knowing if this will work, but intent is already forming. "Onwards, upwards!" he declares.
Speed Of Light takes place in Edinburgh, August 9-September 1; for more information, go to www.speedoflight2012.org.uk and www.eif.co.uk