Lewis Pugh has done more than anyone else to put extreme long-distance swimming on the map.
The 48-year-old endurance athlete has plied the waters of the Arctic, the Antarctic and any number of other seas, rivers, streams and lakes.
He has even done a high-altitude paddle on Mount Everest.
But he describes his upcoming swim off the coast of southern England, during which he will attempt to swim the entire length of the English Channel, as the toughest challenge of his career.
His quest is part of the worldwide Action for Oceans campaign, which calls on governments to fully protect at least 30% of the world’s oceans by 2030.
He says he is ready to risk his life to convince people of the need to protect the world’s oceans.
Pugh will start at Land’s End in Cornwall on 12 July and head east, battling the currents and frigid temperatures for up to five hours a day.
He hopes to make Dover in 50 days but the tides, headwinds, and sheer loneliness of open water swimming may have the last word.
“I’ve swum in some of the most dangerous and coldest waters on Earth but I rank this [swim] a 10 out of 10 for difficulty,” Pugh said.
“I think this is the most difficult swim I’ve ever undertaken, by a long way.”
It is something of a reinvention of that popular feat of endurance known as “the Channel swim”.
Some 1,800 people have done the 33km (21m) crossing between England and France but Pugh will attempt the entire 560km (348m) stretch of water officially designated as the English Channel.
It is a mammoth undertaking and Sky News joined him in South Africa where he is training.
Unsurprisingly, the daily routine revolved around several hours of cold-water swimming, and we watched him skirt the stormy seas that hug the Cape of Good Hope.
“The water is swirling beneath you, it’s scary,” he said.
“And you know courage is a little bit like a muscle – you need to be swimming in these conditions. This is the best training.”
This practice session, like several others we witnessed, was both a physical and a psychological test but Pugh’s preparations have not been limited to the water.
He has taken advice from the youngest captain in the Royal Navy, Matt Syrett, whose polar supply ship the HMS Protector was docked off the coast of South Africa.
Captain Syrett was the navy’s former hydrographer – or chief map maker – and he knows the coast of southern England well.
“Look here off St Aubin’s ledge,” said Captain Syrett, pointing at a number of minuscule symbols on an oversized maritime map.
“You can see the rough water [warnings] indicating the overfalls – and here these symbols are whirlpools and back eddies.
“These are the most dangerous in my view.”
Syrett was also mindful of the mental challenge faced by Pugh: “Inside the Solent there will be a sense of euphoria,” he said.
“Lots of people, in the middle of summer, and I’m sure you’ll see your maximum levels of support because people can get close to you.
“But once you pass Selsey Bill (headland), the coastline changes and you’ve still got an awful long way to Dover and your finish line.”
A key part of Lewis Pugh’s training regime will take place at the kitchen counter, for he also needs to eat.
Like the Cape Fur seals which populate the South African coastline, Pugh requires fat to keep himself warm and fortunately for this endurance specialist, a healthy appetite comes naturally.
We watched him consume a bowl of yoghurt, berries, toast and jam, plus a plate of ostrich meat for breakfast – all washed down with cups of coffee – and when our film crew kept him waiting, his wife warned: “He’s like a polar bear when he’s hungry.”
Pugh said the layer of insulation would not last long off England’s southern coast but it was absolutely crucial to have it.
“I want to have a decent layer of fat so I’m going to go in there at about 100kg.
“Then over the 560km, and what we think will be 50 days, I’m going to get thinner and thinner. It’s almost a race against time.”
Lewis Pugh will suffer in pursuit of his goal.
It is a mighty ambition and he needs the British public’s support.
:: Follow Sky News from Thursday 12 July for updates on Lewis Pugh’s progress
:: Sky’s Ocean Rescue campaign encourages people to reduce their consumption of single-use plastics. You can find out more about the campaign and how to get involved at www.skyoceanrescue.com