The following correction was printed in the Guardian's Corrections and clarifications column, Friday 26 March 2010

In a panel accompanying a report about a solo swimming attempt on the Atlantic we referred to an earlier successful crossing in 1998 by a 31-year-old Frenchman, Ben Lecomte, the first person to achieve the feat. We inadvertently suggested that he took a surprisingly relaxed approach to the task by swimming "in a wetsuit and slippers". Flippers, in fact, were his more practical choice

An unemployed teacher from Peterborough stood in scanty trunks and prepared to dive into the murky waters of the Serpentine in Hyde Park, London. Japanese tourists started snapping photos; even the ducks seemed intrigued, raising their heads from their wings to see what on earth the 22-stone man on the lakeside thought he was doing.

"Lovely, isn't it?" boomed Dan Martin as he padded towards the water in flip-flops with holes in the bottom. "Still, better than last week. I cut my finger on some ice when I got in."

If plunging into a 1C lake at the tail end of the worst winter in more than 30 years seems a daft thing to do, wait until you hear what Martin has got planned for 8 May.

That Saturday, the 28-year-old will don the same pair of Speedos, cap and goggles, wade into the sea in Nova Scotia, eastern Canada, and start swimming. With nothing but his ginger beard and a whole lot of chutzpah to keep him warm, he will head out into the Atlantic, over the grave of the Titanic and up towards the Eurasian tectonic plate.

If all goes to plan and the sharks, jellyfish, trawlers or prolonged exposure to cold water don't get to him, somewhere between four and six months and 3,500 miles later he will emerge in Brest, north-western France.

There Martin will keep going. On a bike. Until he reaches Uelen, Asia's most easterly settlement in Russia. To get there, he will have pedalled through Siberia in the sort of weather that will make Hyde Park in early March seem like Death Valley.

"People think swimming the Atlantic is going to be the hardest part, but it's probably the cycling," said Martin as he prepared to dive into the Serpentine with the nonchalance of a man about to have a relaxing bath. How so? "It can drop as low as minus 80C in the Siberian winter. Ha ha! I don't even like cycling."

When Martin reaches Uelen he will be on the home straight. The only complication is that the straight is rather long. And he is going to run it: one marathon a day, Eddie Izzard-style.

Only when he reaches New York – probably at the end of 2011, when he will be 30 – will his journey be over.

He calls it the Global Triathlon because he is using the whole of the northern hemisphere as a globe-sized triathlon course. But what's the point? "I always say it's just to impress girls in bars, but it's raising money for a great charity that helps orphans in Nepal," he said. "But also I think it's the great British adventure that can be done, and should be done."

He stresses donations will not go towards financing this folly. The estimated £200,000 cost of the trip is being met by corporate sponsorship, so any public money goes straight to the Dan Martin Foundation, his own charity for under-privileged children.

Ever since he went public with news of this adventure, he has received messages on a daily basis suggesting he will die doing it. To avoid that, Martin has been in serious training. Not that you would be able to tell. "At 6ft 5in and 22 stone I'm not your average athlete," he said. "It's something I've consciously done. I need to gain weight for the cold, to get as much stuff between my internal organs and the ocean that's trying to kill me. I think I'll need 7,000-9,000 calories to sustain me each day and odds are that isn't going to happen every day at sea. So it's a calorie reserve and it helps with the buoyancy. The benefits of being chubby far outweigh the downsides for the swimming leg."

Before Martin dreamed up this challenge – after returning from a 22,000-mile cycling trip from South Korea to Cape Town – he weighed around 14 and a half stone. In order to gain the weight, he says he has been doing "everything Gillian McKeith says you shouldn't". In the main, this involves putting away at least seven meals a day and guzzling calorific drinks, including beer. "It was quite enjoyable early on but now I'm training eight hours a day it's difficult to keep the weight on. It's hard to shove enough food in my face," he said.

"The plan is to lose about a stone and a half every month for the first 10 months through the swim and the cycle so that I'm down to about 12-and-a-half stone for the run."

At the Serpentine Swimming Club after his training session, the hardy club members quiz him on his preparation. "You've done the Channel, then?" asks one man. Later, I ask him why not. "It's really hard. All those currents."

• To sponsor Dan Martin, visit www.danmartinextreme.com

The nitty gritty

Has anyone swum the Atlantic before?

Yes. In 1998, Ben Lecomte, a 31-year-old Frenchman, became the first person to achieve the feat, completing the course in 73 days. But he did it in a wetsuit and flippers – the wuss.

What stroke will Martin do?

"Butterfly! Only joking – front crawl all the way," he says. "It's by far the most efficient stroke."

When will he sleep?

After eight hours of swimming every day, Martin will touch his support boat and the skipper will mark the spot on a GPS device. The next morning, the boat will return to that exact spot and the challenge will re-commence.

What about going to the toilet?

"There's an easy way to go to the toilet in the sea, so I'll be polluting ever so slightly," says Martin.

And eating?

To consume the 7,000-9,000 calories needed each day, Martin will drink energy drinks "and maybe a Mini Roll or a banana".

What about sharks?

He is going to wear a shark-repellent anklet to ward them off, and will not swim at dawn or dusk, shark feeding times. "I'm more worried about a Portuguese Man o'War," he says. The jellyfish-like creatures have tentacles that can reach up to 50 metres.