Europe's toughest trek: Corsica's GR20
"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?