Across deserts, oceans and mountain ranges, a new kind of holiday is gaining ground – where pushing yourself to the limit is the whole point.

We are swimming 3km from Cape Pelegrin, a headland on the north-eastern corner of Croatia's Hvar island, across an open water channel to the island of Palmižana. The water is choppy, Palmižana just a distant headland above the waves. When we reach the other side there is a further 5km to go – tracing the indented coastline of the Pakleni Islands, past hidden coves and quiet pine-fringed bays. But I am not looking at the scenery, I am focussed on the pink tow floats of the swimmers ahead.

In peak summer, this archipelago is filled with day-trippers from Split, lounging on boats and floating in the turquoise shallows. But in early May, the beaches are empty, the water is cool and the only sound is the rhythmic splash of arms breaking the surface. I'm here to take part in UltraSwim 33.3, a four-day, point-to-point adventure across Croatia's sun-splashed southern coastline, covering 33.3km – the equivalent distance of an English Channel crossing. UltraSwim is part of a growing trend in travel: endurance-based holidays where the goal isn't relaxation but transformation.

"I wanted to create something that's a cross between a race, a challenge and an adventure holiday," said founder Mark Turner. "There's a generation – people aged 40 to 60 – who still want that challenge, but also want to stay in a nice hotel, eat well, maybe bring their partner along and explore the region. The Channel is an iconic swim, but the experience itself isn't that nice. What I've tried to create is the same sense of achievement, but in clearer waters with incredible surroundings – and a glass of wine at the end."

UltraSwim is just one example of a growing wave of endurance-based travel experiences that are redefining what it means to take a break from work. Rather than lounging on a beach or exploring a new city, more and more travellers are opting to push their limits – whether that's a 171k ultramarathon around Mont Blanc, running 250km across the Moroccan desert or cross-country skiing 220km through Finland's Arctic Circle.

Alison King, a 56-year-old landscape architect from London, is one of them.

"I decided to enter the Ultra 33.3 swim in Croatia, because I was hungry for more life after two decades bringing up my two children and being tied [down] by schools and routine," she told me.

King, who had never considered herself particularly athletic, was initially daunted by the 12km swim on the longest day. "I liked swimming but I'd never done team or competitive sports. At first, I felt like an imposter. But I absolutely smashed it. I finished strong, calm and elated. It was scary at times, but the joy of being out there in the world, connecting with others and to the ocean – that's something I'll carry with me. It wasn't just a holiday. It was a reset."

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