I am drinking muddy coffee in Zaragoza, waiting for the train to Pamplona. It is 4am on Saturday and there is nearly an hour in hand. From beyond the pale rooftops comes the cacophonous reveille of the city's young: the clanging of metallic drums, the smashing of glass, and discordant chanting. This unusual cork-popping is both permissible and necessary, lest we forget that today, July 6, opens a week of unadulterated revelry in northern Spain, to commemorate a medieval Christian martyr by the name of Fermin. An inebriated Spanish adolescent, tight-topped and greasy-haired, buys a ticket and sits down next to me. Why are you going to Pamplona, I ask. "To defy the bull and, possibly, to die gloriously," he says.

This final leg of my journey follows weeks of intense and, one suspects, pointless preparation. My fitness has never been better and my side-step, carefully honed by hopscotching the piles of dachshund crap in Holland Park, is immaculate. But it would be churlish to believe that any amount of training or study of bovine psychology or, indeed, note-taking during key scenes of Gladiator, will assist me in this most masochistic of human activities. Ultimately, if you choose to run with the bulls at Pamplona, you relinquish all sense of personal responsibility. No one in their right mind would attempt it voluntarily, which is why most runners drink away the night before their 8am meeting with death's personal assistants.

Some 36 hours earlier I had been at the Spectator magazine party - another event at which an alarming amount of bull is released into a vast and drunken crowd in a disproportionately tiny space. I conducted a brief survey as to my potential success in Pamplona. Tim Yeo, the shadow culture secretary, gave me a 10% chance of survival; the cartoonist Kipper Williams agreed to concoct a humorous taurus-motif-on-coffin cartoon should I fail to return. Sarah Willetts, the artist wife of David, MP, shrewdly suggested that I stay in England and make up the article. The editor, Boris Johnson, raised an eyebrow and asked who I was and what I was doing at his party.

With only a few minutes until the Pamplona train arrives, and with the station full of peaky young men armed with bravado and tequila, some variation on Sarah Willetts' advice begins to seem appealing. One thing is certain: with the pandemonium of Pamplona shortly to descend, and with the tightness of my deadline, what follows will either be a fabrication, or the drink-distorted ramblings of a man in the midday sun, who has recently avoided bodily union with a stampeding beast, and quickly developed a lasting and wholehearted gratitude to dachshund faeces.

At 07:55, minutes before a rocket will announced the introduction of six bulls on to the cobbles of the narrow streets, it starts to rain. Behind me a Scotsman, holding a flask of whisky in one hand and a can of beer in the other, chuckles. I tighten my grip on a rolled-up newspaper, my version of the paper weapon traditionally carried to fend off the bull, and attempt to chuckle back.

I position myself midway up the only straight stretch in the enclosed course, for I had heard that from this position, with a degree of luck, I would be able both to run alongside the bulls and, together with the first couple of hundred entrants, make it into the stadium. I repeat my tactics to myself: do not sprint off up the street in blind terror when the rest of the crowd does; wait 90 seconds. When a galaxy of camera flashes emanates from the balcony-bound spectators overhead, start walking; upon the sound of thunderous hooves, jog; and, finally, when a blur of black moves into vision, sprint like hell, side-stepping anyone who has fallen or is in the process of being gored. Do not look back.

A second rocket tells us that the creatures have left their pen. Ahead of the black fighting bulls comes a terrifying pack of steers, known as "quiet bulls", a term which should surely be revised. The crowd, dressed all in white with red scarves and sashes intended to distract the bulls, attempt to scarper into any hiding place that might present itself. They are foiled, though, since there are absolutely no hiding places in Pamplona's encierro, or bull run.

There are various points at which the agile can leap through wooden barriers but otherwise, it's every man (and the occasional dissident, law-breaking woman) for himself. The steers crash past, trampling several underfoot just in front of me. I keep to the side, walking now, and glance at my watch. Eighty-five seconds since the rocket. I'm wondering whether the bulls are famed for their punctuality when the gargantuan beasts come, heralded by a savage roar from the spectators and lightning streaks from the photographers. I break into a jog, hopping over a motionless body. Scarcely the time to be resting.

My run lasts about 150m and is the most visceral experience of my life. In those moments, with a bull a metre from you and people diving for safety all around, you smell the air more keenly. The bull, to die inevitably in the evening fight at the hands of men with sharp implements, has this chance to return the compliment.

There is nothing rational about such intentional proximity to a killing machine, primed from birth to inflict horrific wounds and to withstand enormous physical torment. And yet, as I round the corner on to the slope leading to the stadium, and as the passionate bull by my side slides slightly, as my side-step fails me and I tumble on to my face over another prostrate body, I come close to understanding why people place themselves in such danger. It is the need to offset the inevitabilities of life, experienced most poignantly by the bull, with an action, three minutes long, that heightens your sense of the risks that make life interesting and diverse. It is an affirmation of life's vim.

Perversely, my first response to finding myself on the cobbles is to grab my newspaper. Encouraged by the screaming spectators, I leap up and follow the bull, racing for the light of the stadium. And there, sprinting on to the sand, I feel like a hero for a second, with the thousands sharing the sense that some kind of victory has been achieved. A third rocket signals that all the bulls have reached the ring, and we exhilarated strangers hug each other.

The bulls are herded away and replaced by a succession of three smaller bulls, which chase the runners round the stadium and which, occasionally, are wrestled to the ground by the sheer force of numbers. This is a transition between the running and the fight, in which the authority of man over bull is asserted: once the bull is overcome by the bare hands of the mob, it is led off.

I hobble off after a while and my bruises are attended to by a spellbinding Spanish nurse. "Now listen," she says, looking me in the eye, "next time you really must be more careful."