Just a bit further along the route, which traces Corsica's mountainous spine from north to south, everyone is wondering whether it should be called a footpath at all, and who anyone is trying to kid when they talk about a walk rather than a good-to-moderate scramble. If you lack a head for heights, there is only one meaningful, printable expression for it: this is a rock-climb.
Route finding is not a problem, although it would be impossible without the red-white splashes of paint on rocks or trees every few metres; nor does the overall distance of 140kms sound daunting. Instead, it is the combination of vertical ascent and descent - about 19,000 metres - and the exposed, scary aspect of much of the route.
Like many things in Corsica it comes as a surprise, even though, having studied the map, you should have known it was coming. Some of the confusion lies in most northern Europeans' limited sense of geography: "Corsica? Snow? But it's in the Mediterranean, isn't it?" With Mount Cinto reaching above 2700 metres (nearly 9,000 feet) and lots of peaks above 2,000, Corsica usually collects about six metres of snow during the winter. This year it was twice that amount, so it is no wonder there is a bit left in the summer, even if the island is in the middle of the sun-struck Med.
Surviving each day on the GR20 involves ticking off mental markers on a regular basis - reaching a certain pass by lunchtime or getting a mountain hut in view before nightfall (in which case you are probably cutting it a bit fine). But it works both ways: the view back to the coastal starting point near Calvi disappears, apparently for good, after the first big climb.
"That's the last we'll see of that gleaming azure crescent stretching to the imposing Genoese citadel," you think to yourself. But it recurs, both beautiful and irritating, at practically every ridge top during the next four days. Whatever you want to believe, it says more about your lack of horizontal progress than the clarity of the view.
More encouraging are glimpses of big peaks along the way, which make up what the French call the "partage des eaux", the watershed. Many of them, including the distinctive pyramid of Paglia Orba, are just waiting to be climbed, without a pack on, by way of a day's detour.
Highly rated as a challenge, the route attracts two main breeds of walker. The first are the surreptitiously competitive. Their apparently nonchalant, stroll-in-the-park attitude belies their desire to outpace anyone they come across.
Moving even faster between stops, and fuelled by colossal calorific intake, are the gastronomes (invariably French) for whom each ridge top rest is an opportunity to transfer most of the contents of their rucksacks to their stomachs without even considering the view.
But for those that do, there is plenty to see. The few alpine meadows are as welcome underfoot as they are to look at, for Corsica is so rocky and steep that intermediate ground is rare - it is either mountain or maquis, the impenetrable scrubby cover which blankets the low slopes between the hills and the sea. It used to hide bandits, but now conceals a far greater threat to walkers: wild pigs that will not hesitate to demolish your campsite. More positively, the mountains are as spectacular as you could possibly want, closer to fantasy than reality, with extraordinary, jagged ridge lines begging to be explored.
The most extreme section, the Cirque de Solitude, comes just four days into the walk as you head south. Little else is discussed on the trail over the preceding days, and you do not need to be Faraday to work out what might happen if you were caught up there in a storm, when the chains and cables bolted across the sheerest sections of rock come alive with lightning.
Then, just as you start to think it is all rock and towering peaks, you descend to follow a torrential river through a valley filled with colossal pines, then cross a pass leading to the Refuge Mori to be faced with a combination of Lakeland fells and the Cairngorms, with a view down to Porto on the distant west coast.
The next day is another stunner, offering a view of Lake Nino sitting in a mist-blown valley, surrounded by velvet grassland grazed by ponies and cattle belonging to shepherds from a bergerie - a group of summer shelters - a little way along the river.
After Vizzavona, where road and rail bisect the island, the views are softer, and the climbs gentler. The highlight ought to be the pinnacles of Bavella - a rock monkey's paradise - but just reaching that point at which you look out across the island's south-eastern coastline provides the most profoundly satisfying view of the lot. You really have covered some ground - when you started, you could watch the sun set over the water, and now you are watching it rise.
The sense of being isolated on this jagged, beautiful lump of rock in the middle of the Mediterranean is overwhelming, even if your mobile phone starts picking up Italian signals. Corsica's most famous son, Napoleon, would turn in his grave.
Walking the GR20: The season is defined by snow - too early (before June) and you may face deep drifts, too late and the volume of people and storms can be a problem. Take warm clothing and waterproofs, and a basic supply of food. Trekking poles are vital. For group GR20 walks, contact Sherpa (0181-577 2717) for details or visit: www.sherpa-walking-holidays.co.uk
Maps and guidebooks: Get hold of Didier Richard 1.50,000 number 20 (northern Corsica) and 23 (south), IGN 1.25,000 topographical, The Corsican High Level Route by Alan Castle (Cicerone), Walks In Corsica (Robertson McCarta) and Corsica Mountains by Robin G Collomb (West Col Productions). Lonely Planet's Corsica includes a comprehensive section on the GR20. The first 20 readers who write to: Independent/ Corsica Competition, Lonely Planet, 10a Spring Place, London NW5 3BH win a free copy.
Getting there: There are no direct UK-Corsica flights. Air France (0181-742 6000) can offer connections via Paris. For an air/sea combination, use EasyJet (0990 292929) from Liverpool or Luton to Nice (as little as pounds 107 return), transferring to domestic flight to Ajaccio or taking an SNCM ferry. Or, a Eurostar/TGV return rail ticket from London Waterloo (from pounds 129 booked through Rail Europe, 0990 848848), then a ferry.
More information: French Travel Centre, 178 Piccadilly, London W1V 0AL (0891 244123, a premium-rate number).Reuse content