My friends think of me as a cyclist. I've always been inclined to agree: I'm not afraid of the big roundabout at Elephant and Castle, in South London, and I go by bike even when it's raining. So when my friend Jonny asked me in November if I fancied doing a stage of the Tour de France the answer seemed obvious. I like cycling and I like the Pyrenees. What could be nicer? In April I did the London Marathon. I was in good form.

In May, with two months to go, Jonny suggested it was time for a proper training ride, so we entered something called a sportive - an 85-mile jaunt over the South Downs. Everyone gets a timing chip, signs to show the route, and some orange-quarters and flapjacks along the way. It sounded fun. Like a sponsored walk but without having to pester your relatives.

The car park was full of middle-aged men in tight-fitting clothing. They were making fond adjustments to bikes that had cost two months' wages and possibly a marriage. They traded banter about headsets and hubs and filled specially designed pockets with caffeinated energy gels. These people were cyclists, and I was absolutely not one of them. I didn't even have padded shorts.

At the first food station I feigned a minor mechanical problem to let the nausea subside. After 60 miles I was seeing dots, and I devoted the journey home to dreaming up reasons, professional and personal, why a trip to France in July would, sadly, be impossible. I had entered for fun, but now it was not at all clear where the enjoyment would be coming from. If golf is a good walk spoiled, cycling over the Pyrenees was looking like a beautiful hike totally ruined.

Every year the organisers of the Etape du Tour pick one stage of the Tour de France for amateurs to have a go at a week before the big boys get there. The 2008 route covers slightly more than 100 miles from Pau to a ski station called Hautacam, passing over the mighty Tourmalet, one of cycling's legendary sites. For those not making a strict cut-off time there is the broom-wagon, a bus that scoops you up and takes you home. The French call it répatriation, as if your failure will bring national shame and a phonecall to the Queen.

In recent years, apparently, some riders have used drugs so potent that they've had to get up in the middle of the night for a few press-ups just to keep their hearts going.

I planned a less controversial approach: a bit of training and some porridge on the day. For a Londoner, the training poses a problem. The total ascent of the 2008 étape is about 3,300 metres. That means cycling from sea-level to the top of Ben Nevis three times. One evening, feeling more panicked than enthusiastic, I rode up the hill between Dulwich and Crystal Palace until I couldn't face it any more. It was hardly the sustained grind of a true mountain climb, but it was as mirthless a session as I imagined the real ride would be.

There was, indeed, very little mirth in evidence in Pau at 6am on the day. It was raining not hard but with real stamina, and the motorway exit was clogged with 8,000 riders all tetchy and precious about their gear indexing and race nutrition. And it is definitely a race. You don't wear clothes like that without wanting to beat a few people. Cyclists sully the purity of the mountaineer's mantra: “Because it's there, and because I have a bike which is better than yours and designed by Nasa.”

I got my steel-frame bike second-hand from a manic depressive who must have ordered the paint job on an up day - it's like a 1980s drag-racer, all Technicolor flames. Having a steel bike among all those carbon-fibre and titanium machines was like turning up for a Formula One race in a much-loved MG: sort of admirable, and sort of dispiriting. I had left my rear reflector on, too. There were as many rear reflectors as there were pairs of stabilisers. To stave off mountain-top hypothermia I'd bought a strange garment called “sleeves”, which are just that. Simple as they are, I failed to master their use, and for the first 60 miles they kept slipping halfway down my arms to become evening gloves, making me the only competitor in burlesque fancy-dress. To cap it all, an administrative glitch meant that I had been registered as Jean-Francois Lardy, a no-show. He must have read the weather forecast.

And yet from the moment we set off, 1,000 at a time in a determined drizzle, I was buzzing. To cycle in a huge pack free of traffic lights and cars is to feel the elation of kids who have taken over the school and are running wild. Add a tailwind, and 80km glide past in two blissful, rain-sodden hours.

The climb to Tourmalet's 2114m top starts gently enough, but 24km of it are in earnest, at gradients of 8-13 per cent. In cycling parlance the climb is rated hors catégorie (unclassifiable). On a slope like that a bike can start to seem like an absurd contraption, a hindrance. The herd rode in an intense, unshakable silence that took me straight back to the exam hall at school. Thick cloud obscured a view that nobody had eyes for. A grim-looking ski resort inched past. At the summit I swapped exhaustion for fear: my hands were so cold that I couldn't rely on them to brake, and my bike was wobbling as if the frame was broken. My body felt stiffer than the steel I was sitting on. It took me 25 minutes of terrifying descent to realise that the only thing making it wobble was my trembling. Jonny and I crept down like teenage girls while hundreds hummed past hands-free, cheerfully munching energy bars.

When things levelled out I had to teach my legs to pedal again, and they took me hurtling fabulously fast down a curving gorge. I thought the flat section leading to Hautacam was lonely until I looked behind me and saw a train of 40 riders who had been enjoying my slipstream. With a true pro gesture, I motioned the man behind me to overtake with a twitch of my elbow, and we rode more equitably to the village below Hautacam, where we left the only glimpse of sun all day for more freezing gloom and a final hour of uphill grind. The whole thing took us 7 hours. A short day in the office.

A week later Jonny and I watched metre-by-metre coverage of the real stage on a geeky sports channel. It took them 4 hours. They hit more than 96km/h on the big descents, and on the climbs spectators running alongside couldn't keep up. But they weren't smiling as much as us.