There are several ways to react when a friend suggests you take on something described as "Europe's hardest long-distance trek". A straightforward refusal, for example, or a plucky pledge to give it a try once you've trained sufficiently – say, for five years. It takes a particular mix of ignorance and bravado to instead shrug a shoulder and nonchalantly ask: "How bad can it really be?"
One hundred and seventy kilometres traversing Corsica's steepling spine. Nineteen thousand metres of ascent and descent. Fifteen days walking for a minimum of six hours a day. These numbers seemed abstract when we decided to take on the Grande Randonnée (GR) 20. To men of greater trekking experience – the sort who wouldn't have had to borrow their rucksacks off mates, or then filled them with iPod chargers and beach towels rather than freeze-dried spaghetti and crampons – they would have served as a dread warning of what was to come.
Not us two. With one straight from a job with a Parisian luxury goods company so cushy he gloried in the soubriquet "Champagne Nick", and the other held together by bandages after foolishly attempting to complete a full decathlon in a single hour, those bare statistics hinted only at charming mountain vistas, heroic tales of derring-do among sun-baked peaks and impressively muscled legs to show off at the next available barbecue.
Two days in and reality has sunk its teeth into our sweaty behinds. In 13 hours of toil, we have yet to encounter a section of flat path that stretches for more than 10 metres. Only rarely have we encountered a path at all. Instead, the red and white daubs of paint that serve as trail markers have sent us scrabbling up nightmarish boulder fields, sliding down vertiginous scree slopes and inching across precipitous rock faces. Walk? There isn't the opportunity. This is rock-climbing without ropes. There are astounding panoramas – don't look down — but we're too busy not tumbling into them to appreciate it all.
"Is this a joke?" gasps Champagne Nick, dropping his rucksack to the ground and falling on top of it. He jabs a finger at a brutal cascade of enormous granite lumps that stretches up and away to the horizon. Having set off at sunrise, we have already hauled ourselves up the equivalent of one and a half Ben Nevises. Strung out down the bare valley below are a handful of fellow trekkers, plodding upwards slowly like pack donkeys. A 900m descent over treacherous loose stones is still to come. How bad could it be? Very.
The GR20 – or "Jhay Air Vang", as the locals pronounce it – does not mess about. From the very first step it climbs at a dizzying rate, through dense maquis forest and then larico pines, past shepherds' huts and hoofprints left by wild boar, up through the wispy coastal clouds and away into the silent, mountainous interior.
In winter it's impassable with snow, in high summer sweltering, which is why insiders recommend late June and early September. And while it's possible to split it in half and do just the north or south sections, the traditional route cuts from Calenzana in the north-west, a 10-minute taxi ride from Calvi, all the way across to Conca in the south-east, returning via dusty bus two weeks later.
It deserves enormous respect. Unfortunately, we have barely given it a second thought. Two thousand metres up at Bocca Piccaia, with grey, jagged peaks the shape of Stone Age spears emerging and then disappearing in the mist below us, we cling to one of the few handholds on the fissured rock face for dear life while playful gusts attempt to nudge us into the abyss. "This is beautiful," I hiss at Nick, "but insane."
There is a happy shout behind us. Three clean-limbed French girls are skipping along the skinny ridge with all the ease of gymnasts on a high beam. Close behind is a middle-aged Belgian man wearing a raver's bandana. The previous night he had introduced himself as his country's biggest retailer of industrial castors ("All sizes. They are amazing things"). He spots us and waves delightedly. "Let's run the descent!" he yells, and bounds off at suicidal pace.
It takes us three more sore-legged hours to catch up with them at the end of the stage. We are ready for hot showers, rub-downs, gargantuan meals and beers so cold they could please a penguin, but this is the GR20. There are no four-star hotels, no charming converted farmhouses to stay in. For the majority of the route, the choice is simple: a mattress on the floor of a tiny wooden refuge at €15 a pop, or a tent pitched in the stony ground that surrounds it.
At first glance these refuges seem charming – reminiscent of the sort of bucolic Alpine hut that Heidi might have stayed in with her grandfather. There are tinkling goats, spectacular prospects across summits and valleys and ancient bearded guardiens to provide you with student-style cooking at extravagant prices. Unfortunately, there are also 25 pairs of stinking walking boots, the Mediterranean's most primeval toilets and sleeping quarters so cramped you may as well lick your neighbours' feet at the start of the night and get on with it.
With lights off at 8.30pm, you'd expect to get your fill of sleep. That fails to take into account the international snoring contests which break out as soon as darkness falls. The only noise that's more disturbing comes around 2am, as 20 sleep-deprived individuals queue outside the single latrine toilet.
Freeze-dried food might be light in the backpack, but it's heavy on the guts. As a chill moonlight lights up the trees overhead, the small wooden cubicle shakes to what sounds like a series of balloons being burst in a vat of custard. Nick glances at the three French girls standing wearily behind us. "I think I might find a private bush," he whispers.
The trek's daily rhythm is soon established. At the first hint of dawn, the refuge comes alive with the blinking of head-torches and the zipping of fleeces. Food is rehydrated and boiled to destruction, weak coffee glugged down, toilets devastated. As soon as there's enough light to see the broken ground under your boots, the little groups set off – some in somnolent silence, some with cheery song, others loudly discussing the Parisian champagne market in a desperate attempt to forget about blistered toes and decathlon-induced injuries.
For at least two hours, there will be an uphill so relentless that it's like being on a mountainside treadmill. What breath is left will be taken by the sort of view normally available only to helicopter pilots and people watching Lord of the Rings on DVD. At some point an eccentric veteran will overtake, wearing high-cut denim shorts and clip-on shades, drinking wine and offering a slice of saucisson as he breezes past. That anyone might attempt a lie-in, or breakfast at leisure, seems unthinkable. The unspoken orthodoxy seems to be that the GR is a beautifully backclothed trial as much as a trail, which makes it both mildly shocking and all the more captivating when we fall in with the Lebanese posse.
Georges, Marc, Raph and Nabil are schoolboy friends out for a middle-aged adventure. While others mix rehydration sachets, they pass round the malt whisky. While others retire with ear-plugs as soon as the sun goes down, they crack out the eau de vie and actually look like they're having a good time.
The Cirque de la Solitude is the most notorious single section of the entire GR20. People are so scared of it that they huddle in petrified groups the night before, exchanging horror stories and then going to bed even more prematurely than usual. Not the boys. They buy endless bottles of Corsican wine from le guardien, pull cartons of duty-free fags from their backpacks and invite us to paint the refuge red.
The Cirque turns out to be both gorgeous and terrifying at the same time. After 800m of rope-free climbing up, there's 300m of straight down – straight down as in sheer rock-face, without even any pretence of a path. As a sop to the cowardly, there are some old chains bracketed into the smooth granite. As a teaser, there's nothing but the occasional spiky outcrop between you and the valley floor 1,200m below. The eau de vie hangover lends it all an air of enjoyable farce. After inadvertently essaying a spontaneous abseil that is a lucky ledge away from full flight, Nick gulps: "That's the closest I've ever come to losing my life," and instead of lighting a flare and waiting for mountain rescue, we all laugh uproariously and clatter onwards.
For all the tribulations, the rewards are remarkable. The terrain is like nothing else in Europe, the set pieces the sort of thing that have even grizzled veterans swooning like knock-kneed novices. Adrenaline overcomes exhaustion, camaraderie keeps the aches at bay.
When we reach the overnight stop at Bergeries de Vallone, a large rock pool is discovered in the nearby river and cans of Pietra beer bought from a shepherd at a mark-up of just 400%. Lebanese charm and free alcohol persuade the French girls to join us for the GR version of a pool party, and as the sun slinks away behind the darkening mountains and water boatmen zip across the flat surface of the river, a sozzled sense of satisfaction comes over us all. The Jhay Air isn't so bad after all, we decide.
In retrospect, it was probably a mistake to accept the bottle of cherry brandy from the shepherd. Whether that alone is responsible for the lung-splitting reworking of the song YMCA as GR20, the midnight javelin competition with our walking poles and the demolition of a giant wheel of fresh goat's cheese we may never know, but things are never quite as good again.
Maybe it's the Lebanese bailing out – like many others, they'd decided to do just the northern half of the trek – or maybe it's the dodgy water source from the refuge at Manganu that leaves us all vomiting for a deeply unpleasant 36 hours. Perhaps it's the unbroken routine of trek, refuge, bad food, poor sleep, trek, refuge. Whichever, with a week and a half gone and no end in sight, a grim, humourless mood of resignation overtakes every walker.
That the landscape gradually softens to a succession of lush oak forests and rolling pastures conversely makes the going harder. While the crag-clambering had been sadistic, the challenge of getting through each stage kept the mind fresh. When it's just a case of slogging onwards, day after near identical day, motivation is harder to maintain. Fatigue fills our rucksacks with lead. Each jarring downhill stride hurts a little bit more. Of those who had started at the same time as us, only the French trio remain en route. Deep into the south part of the island we try to push the doubts away. Having come this far, we aren't going to be among the 75% who fail to make it to the finish.
That's the plan. The Manganu parasites have other ideas. With the downhill run to the coast at Conca just three days away, the vomiting and exhaustion return with a vengeance. Unable to pick up our bags, unable to lift our heads from our knees, we hear the white flag being raised overhead.
"Arse," says Nick glumly. "Stomach," I reply. He looks at me and grins. "Does it count if you finish in a taxi?"
• Tom Fordyce's book (with Ben Dirs) about his attempt to become a sporting world champion, We Could Be Heroes, is published by Macmillan. To order a copy for £10.99 with free UK p&p go to observer.co.uk/bookshop or call 0330 333 6847
Essentials
Tom Fordyce travelled as a guest of the French Tourist Board (uk.franceguide.com). Refuges on the GR20 cost about €15 a night and can be booked through the Parc Naturel Regional de Corse (parc-corse.org). Further English-language route and planning information is available from Corsica for Hikers (corsica.forhikers.com/gr20).
Easyjet (easyjet.com) flies to Bastia, a two-hour bus ride from Calvi, from Gatwick, Manchester and Bristol.
