Why We Run: A Story of Obsession, By Robin Harvie

For most of the 35,000-plus people taking part in today's London Marathon, the event marks a chance to do something truly extraordinary.

A feat of endurance in memory of a lost loved one; an athletic achievement to prove that a dangerous illness is finally in remission; or simply a grand gesture to try to bludgeon impending middle age before its tentacles grip. It's Glastonbury for those with a passion for sweat-wicking fabrics and energy bars: the Big One.

But for a small sub-group of runners, the London Marathon is boring. It's not the repetitive training, the relentless stretching or the frustrations of running for nine miles behind a middle-aged man dressed as a rhinoceros: it's the distance. It's just not far enough. Robin Harvie was part of this group. A distinctly average runner, he completed his first marathon in 2000, and six years later he was still unable to improve on a time of three hours 30 minutes. Frustrated, he decided instead to increase the distance. This is the story of his attempt at the Spartathlon.

Set up in 1983, the race retraces the journey of Philippides, the ancient Greek messenger and herald whose story inspired the original marathons, through the Peloponnese from Athens to Sparta. It's 152 miles, it takes up to 36 hours and there is no cash reward for winning. As far as ultra-marathons go, this one really is the daddy, and yet it's an amateur event. As Harvie explains: "The competitors are the people who guard your prisons, put out your fires, decorate your homes and deliver your post." Fewer than 700 have ever completed it, and Harvie was determined to become one of them.

Nominally, this is a book about extreme running, and there's no shortage of detail on the Spartathlon. From Harvie's extensive preparations, which gradually threaten to overwhelm his "real" life, to the physical toll that it takes on his body, he is as scrupulous as he is lacking in vanity. Feet pummelled by the dirt tracks of the Greek countryside, toenails "popping out" and the hallucinatory effects of running in the heat for 15 hours: the grisly details are all there, and it's hard not to revel in the gore.

But where the book truly excels is in its depiction of Harvie's internal landscape. He largely shuns training tips and inspirational advice in favour of a true memoirist's tone, exploring the reasons why he runs – grief, ambition, boredom – with an almost brutal honesty. These passages are as moving as they are illuminating. It's a runners' cliché that the hard work with marathons is largely mental, and Harvie's tale corroborates it.

He argues that we all need an internal space, a way of shutting out the world, an obsession to call our own, and he's quite right. Training technicalities aside, this is a memoir for anyone who has ever dreamed about reaching the outer limits of what they're capable of and, as such, it should be enjoyed by an audience far wider than just those who head home this evening wearing a medal.