Throwing my pack on the bunk I head back outside the refuge to wind down in the last of the afternoon sun. All of the seats are taken, but a bare patch of ground with a wall to rest against is plenty comfortable enough for my beleaguered body.
I pull my phone from my pocket and send a text message to my Dad. "Ridiculously tough trek. Terrain is brutal. Two days in and I'm already broken." I had been warned this would be the case by the various guidebooks, websites and blogs I read over the past few weeks but dismissed talk of this being the "hardest trek in Europe" as high-altitude hyperbole. I expected Corsica's GR20 to be difficult, but not that bad. Today I was proved wrong. Emphatically. Almost immediately after setting off at 7am, we were clambering over 40-degree slabs of granite, scrambling up boulder fields masquerading as paths, traversing exposed ridges and descending scree slopes that may as well have been made out of marbles.
Twice we undertook enormous climbs up to wind-battered passes and twice we had to stumble all the way back down to the next valley along, our thighs, calves, knees and pretty much every other part of our anatomies already screaming in submission.
The views were magnificent - cathedrals of rock towering over deep pine valleys - but the trail to get to them was quite simply the worst I've ever set foot on.
Now, sipping the dregs from my water bottle outside the Asco Stagnu refuge after more than 10 hours of toil, I'm beginning to wonder if this beast of a hike would have been best left alone.
The only consolation is I'm not the first to feel this way and definitely won't be the last.
Thousands of walkers take on the GR20 each summer, navigating all or part of the 180km between Calenzana, in the north-west of Corsica, and Conca, in the south-east.
Those with any sense can do just the northern or southern half over a week and call it quits at a village in the middle of the island called Vizzavona, where a train is waiting to take them back down to the coast.
But for those like me deluded enough to do the whole thing, an average of 14 or 15 days of walking containing more than 12,500 vertical metres of climbing and more than 10,000 vertical metres of descending awaits.
There are refuges at regular intervals along the way offering cramped beds for the night and unnecessarily expensive meals, but places get booked up months in advance and many instead opt for carrying a tent, stove and countless kilograms of pasta, noodles and whatever other rations they can fit into their packs.
The campers consequently struggle along with 70, 80 and even 90 litres on their backs and, for some, the draining combination of heavy loads and torturous terrain means the kilometres tick by morale-sappingly slowly.
Pressed for time, my friend and I are attempting to complete it in a far more ambitious 10 days, with an 11th added on to climb Corsica's highest mountain, Monte Cinto, and the only way to make that feasible was to book beds early and cram only the absolute essentials into the smallest pack possible, which for both of us was 32 litres.
The smell generated by just two T-shirts, a fleece, a rain jacket, a down jacket, one pair of shorts, one pair of trousers and three pairs of walking socks will no doubt endanger public health when I stride into Conca, but on account of how many struggling people we have overtaken so far, it appears travelling light has been a shrewd decision.
Yet rushing also has it cons. The GR20 is made up of 16 'stages' varying from two and a half hours to seven, and while almost everyone 'doubles up' on one or two, we're doubling up on no fewer than six stages, four of which our guidebook predicts will be 10-hour-plus slogs.
Breaking point Today has been the first of those epic walking days and even though the dust is still settling on both my boots and my thoughts, I'm already regarding this as one of my hardest days of walking ever.
With dinner devoured and my wallet significantly lightened, I'm asleep as soon my head hits the pillow but we're up again early the following morning for our attempt on the 2,706m Monte Cinto.
It's not on the route of the trek but my semi-masochistic friend thought it would be a good idea and, in the name of naivety and bagging a peak, I decided not to protest.
I'm soon regretting it. The boulder-covered trail to the top is somehow even more broken than the GR20 and our progress is consequently so slow that after four hours of lung-busting, quad-crunching slog, we're still only at a small plateau well below the summit.
We stop for a drink, snacks and photos of the chaotic yet idyllic valley up which we've just climbed, but as we do so, the wind whips up and dark clouds suddenly muster over the mountain's craggy pinnacle.
The temptation to press on regardless is hard to resist, but recent events have made us all too aware of how dangerous these peaks can be in bad weather.
Just a few days earlier, seven hikers had been tackling the most difficult section of the whole GR20, the notorious rock walls of the Cirque de la Solitude - not too far from where we are now - when heavy rain caused a landslide and washed them away to their deaths. Several others were hospitalised but survived.
With the tragedy fresh in mind, the weather worsening and time getting away from us, we decide to wave a white flag at Monte Cinto and turn back, taking solace in the fact we don't have to rush the descent and can at least enjoy the humbling beauty of this part of the range.
We are due to tackle the Cirque the next morning - our third day on the GR20 proper - but it's still closed and so instead we head to a road adjacent to the refuge to catch a bus around it, rejoining the route in the next valley along.
Within a couple of hours of picking back up the red-and-white daubs of paint that mark all 180km, the GR20 resumes its quest to snap our bodies and morale by hitting us with the nightmarishly steep scramble up to the 1,962m Bocca Foggiale.
The ascent is so vertical in places that you use your hands as much as your feet to gain ground and by the time I finally stumble my way to the top 90 minutes or so later, my fingertips are raw and my shoulders feel like I've rowed the Atlantic.
Thankfully, the end of the stage is waiting not too far away at the Refuge de Ciottulu di i Mori, which sits in a majestic spot at the throat of an expansive grassy valley, overlooking neighbouring peaks in the foreground and the Mediterranean in the far distance.
While refuges in the Alps are wonderfully atmospheric places that can be as much a highlight of the holiday as the walking itself, here on Corsica they are far more rudimentary.
From the outside Mori's stone facade gives it a quaint appearance, but on the inside it's bare and basic; not an ice axe hanging from the wall or a black-and-white photo of triumphant 1930s mountaineers to be seen.
And space is at just as much a premium as nostalgia. During the busy summer months it's a scrap to claim a seat in the dining room and then so many people get packed into the tight rows of bunks that the chances of exchanging midnight cuddles with the stranger lying next to you are so high that you may as well introduce yourself and apologise in advance. When it comes to finding your boots in the dim light of the morning, well, good luck with that.
The food isn't a great deal better. Three-course meals are available for 20 euros, but the guardians who prepare them aren't exactly Michelin-starred chefs. Put it this way; you're not likely to be asking for the recipe afterwards.
But then there's a certain rugged charm about the GR20's refuges. Cosy and comfy just wouldn't quite fit with this walk, and anyway, you're usually so tired and hungry by the time you reach them at the end of a long and savage day that even a bed by the draughty door and a bowl of salty lentil soup feels five star.
The GR20 falls into a rhythm over the next few days: wake before dawn; eat a badly crushed cereal bar I brought from home for breakfast; start walking at first light; climb up an enormous hill; take photos of fabulous vistas at the top; try not to break an ankle on the descent down the other side; then go up and down some more hills until we reach our night's refuge.
Race to the finish The seventh day is thankfully scheduled to be only four hours long, but with my friend's penchant for punishment somehow not satisfied, he decides we should go off-piste once more to tackle the 2,622m Monte Rotondo before starting the stage.
Yet again I neglect to protest, only this time I'm glad I don't, because although it's difficult to follow the trail, the climb is a joy.
The highlight comes 200 vertical metres from the top, when a picture-perfect lake known as Lavu Bellebone suddenly appears with the summit ridge standing just behind like a castle wall. We had no idea it would be there and it consequently feels as if we've stumbled upon an undiscovered wonder of the world.
The 360-degree views from the top prove just as spellbinding and, given that no one else has chosen to make the ascent today, they're ours to enjoy alone, with not even the wind bothering to pester us.
For the remainder of the day and all of the next, we make our way back down to the GR20 and then along to the halfway point of the trek in Vizzavona, where we gorge ourselves on pizza, beers and soft drinks in the village's limited choice of restaurants.
We need the calories, because our relentless schedule means we must knock off the remaining six stages and 92km in just three days, all of which will be a minimum of 11 hours in duration.
We take confidence from the fact that the GR20 supposedly flattens out slightly in the less mountainous southern half, but we quickly find that although we're no longer crawling up and down colossal peaks, it's still far from a Sunday stroll and the going remains tough.
At least there are proper paths for the first time - as opposed to the incessant scree and boulders of the north - and so we make good progress along forested trails on our third-to-last day. Our penultimate day lifts us steeply back up on to a long, serrated ridge and our pace drops again, but unbroken views of the southern part of the island and the Mediterranean further afield are more-than-ample compensation.
By the final day my feet are in total turmoil - desperate for a pair of flip-flops and the sanctuary of soft sand down at the coast - but the spectacular scenery helps take my mind off the pain, as first the Bavella Needles and then the increasingly red rock of the southern peaks ensure the GR20 finishes off with a flourish.
When we finally limp into Conca, punch-drunk and stinking like farm animals, it's perhaps fitting that there is no finish line, no obvious point to take a victory photo and pump a triumphant fist in the air. You don't conquer the GR20; rather you merely experience its many challenges and treasures and hope to get to the end. Hardest trek in Europe? It gets my vote.
