As I gingerly wade into Lake Windermere on a cloudy afternoon, I can't help but wish that I had chosen a better day for my first swimhike. Peter Hayes, a floppy-haired university lecturer and swimhiking enthusiast, is already luxuriating in the water, trying to convince me that it is pleasantly mild. He is either lying or he's done this so many times that he is immune to the cold. "Our route for today is to start at the widest part of the lake and swim to a nearby island, before getting out and having a little hike, then swimming back to the start," says Hayes.

It looks a bit far, but I nod and follow his lead. Hayes has been swimhiking since 2002, when he decided that combining his two favourite activities, swimming and hiking, was the perfect way to get fit and explore the countryside. "With swimhiking you are freed from the constraints of hiking. On foot, rivers and lakes become a barrier, but if you swimhike you can go much further, or at least in more directions," he says while doing front crawl.

Hayes began jotting down details of his favourite swimhikes around the Lake District. A total of 44 routes have now been published in a book, ranging from gentle excursions to challenging and invigorating journeys such as the Frog Graham Round, which includes a three-mile swim and covers 40 miles in all.

In order to switch between land and water, Hayes came up with the idea for what he calls a swimsac - a rucksack with a waterproof inner bag to keep your clothes and valuables dry. "It's a low-tech thing," he says. "I've only made a few prototypes, but it means I'm not tied to the spot where I would have left my clothes. Dogs would do their business on my clothes and kids would steal them, though the worst was when a public-spirited citizen saw my bundle of clothes and told the police I had committed suicide." Hayes' website provides instructions for making your own swimsac.

With my swimsac on (it ties under the arms so your strokes aren't restricted, and has inflatable floats in the side pockets for buoyancy and balance), I swim fast to catch him up, moving away from Millerground, today's starting point, towards Rough Holme Island. It takes a while to get used to the chilly water and people on the shore laughing and pointing at us, but that's soon overtaken by the beauty of the surroundings. Open-water swimming feels really liberating.

However, being at one with the elements also means concentrating on not swallowing the flies that hover around the water's surface, while keeping an eye out for anyone doing watersports. Hayes also warns me that occasionally seagulls dive down right behind him and screech loudly, though it's the man-made dangers, such as broken glass, which pose the greatest threat.

We're well on our way when the sky turns even darker and plump raindrops start pelting our foreheads. "Swimhiking is perfect on a hot summer's day," says Hayes. "You build up a sweat on land then cool off in the water. But I also think it's fun in the rain. When you're walking and the raindrops hit your clothes it's frustrating, but if you're in the lake, there's water all around anyway so it feels natural."

The weather gets progressively rougher, though, so Hayes decides that we should ditch the island idea and hike up a nearby hill, bringing us back to the starting point. He identifies a place ahead where we can get out safely.

Back on land we rummage in our swimsacs for our towels and clothes, and soon we're off again (naturally I feel more like going for a post-swim cup of tea - swimming works every muscle in your body). "What I love about swimhiking," says Hayes, "is that you are mixing land and water, so you feel a bit like James Bond and Tarzan."

We march up through the woods and eventually emerge at Queen Adelaide's Hill. The view is spectacular, sheep are grazing by my feet and the sun starts to pierce through the clouds. After a short rest to greet some friendly fellow hikers, we head back down through the undergrowth, watching out for low branches while keeping a constant pace. Hayes dips into his bag and pulls out a map to double-check our location, before leading us back to our starting point.

Even though we had to shorten the route on account of the great British weather, the mile and a half of hillwalking and swimming leave me exhausted but with a sense of achievement. Being outdoors, inhaling fresh air and watching the birds land on the water as you plough through it was a glorious experience. Kate Rew, author of Wild Swim and founder of the Outdoor Swimming Society, agrees: "It makes people really happy. There's something about bobbing around, free from gravity, in wide-open spaces. It doesn't cost a thing and it gives you the chance to see the country in a fresh, new way.

"Most people find that they can only stay in a pool for 20 minutes before getting bored, but up to an hour or more if they are swimming outdoors. If you then combine that with hiking, you're getting a real cardiovascular workout".

For more information visit swimsac.co.uk

· Swimhiking in the Lake District and North East England by Peter Hayes is published by Gilbert Knowle at £9.99.