He is one of the most recognisable faces in the world of freediving, more accustomed to the silence of the deep and slow descents along a guide rope than to mountain trails. But last weekend, Guillaume Néry, a Café du Cycliste Caravan athlete, swapped his monofin for trail-running shoes. The Nice-born champion lined up for the Mercantour Skymarathon, a 50-kilometre course with 4,148 metres of climbing, and came away full of wonder.

Talking to his partner, Audrey, at a feed station halfway around the course, Guillaume summed it all up: “I'm having the race of my dreams.” Two days later, sore but clear-headed, Guillaume confirmed that this edition would remain, for him, an almost perfect experience. Much of that came down to the setting. "Every climb took us onto summits with commanding views across the Mercantour massif. It was extraordinary," he said. From exposed ridgelines to high-altitude lakes, passing through forests that plunge towards the Boréon valley, the Skymarathon offers a concentrated taste of the Alpes-Maritimes, for the trail-runner who perhaps is searching less for pure performance and more for a genuine mountain experience.

For several years now, Guillaume has "stepped back from pure freediving" – the world of guide ropes and extreme depth – but without loosening his connection to the sea. "Spending more time in the mountains makes me want to return to the sea even more," he explains. For him, the sports complement rather than replace each other. Running, which he discovered three years ago, has become the ideal tool for winter training: efficient, demanding, and technically far richer than it first appears. "What seems like a simple movement actually involves a huge amount of technique and endless room for improvement," he says. He's grown to enjoy road running too, tackling both 10km races and marathons. Trail running, meanwhile, has become his spring and summer discipline: gaining altitude, seeking cooler temperatures, and exploring bigger landscapes. "What fascinates me is that, when you run a little faster, you suddenly have access to much greater distances," he says. It's a mindset he compares to cycling – again a sport of travel, and a constant thread through his life. According to Guillaume, cycling's sensation of flow remains "unmatched", and close to freediving itself. Unsurprisingly, cycling now forms a natural part of his trail-running preparation, just as it once did for his freediving training.

Behind the emotion lies work. After a winter focused on road running – five to six months of preparation built around five running sessions and two strength sessions each week in preparation for a 10km and a marathon – Guillaume shifted his focus to trail running: three to four trail sessions, one bike ride and two strength and conditioning workouts every week. He does it all without a coach, designing his own training cycles. "I've built everything around feel, experimenting and trying to find my own path," he says. The man of the deep has brought the same methodical curiosity to the mountains that once shaped his freediving career. The verdict? Excellent feelings throughout the 50 kilometres, no injuries despite pre-existing meniscus fissures and a nutrition strategy based on "the most natural foods possible", far removed from carefully measured gel protocols. "It worked unbelievably well," he says.

There was another distinctive aspect to his race: he had entered as part of a group of three, with friends Léa and Maxime. Over such a long distance, a shared adventure has several different forms. The first 25 kilometres were run together, at the same pace, without anything having been planned. Then their paths diverged. Feeling particularly strong on the formidable second climb, Guillaume pushed on alone and spent two hours by himself during the descent from Mont Archas to the Boréon. “I loved that time on my own," he says. "That's what's so special about races this long: you live through several different races within the same race.”No race involving 4,148 metres of climbing passes without a low point. His came just after the Boréon, where Audrey and his son Kaï were waiting for him: a very difficult hour followed, particularly on the third climb, up to the Cime du Piagu. It was there that Léa, having come back through the field after Maxime got into difficulties, caught up with him. At the summit came the reward: the late afternoon light falling across the peaks of the Mercantour, as far as the Cime du Gelas, the highest peak of the Alpes-Maritimes. “It was sublime,” says Guillaume.