The journey included a spell sleeping on the floor of the club where Barry coaches, Tideway Scullers School in west London, after leaving the Army.
Barry recalls: "I got a phone call saying, 'There's a mad guy down here who looks awful but he keeps winning the Wednesday night sprints. Can you come and have a look?' It was Alan.
"I asked him what his objective was and he said, 'To win in Beijing'. It may have been a little tongue-in-cheek at the time but it fitted with my own mission to find a single sculler to win Olympic gold.
"I told him he needed to lose 10kg because he was too fat and off we went - that was five years ago."
A year later, at the age of 21, Campbell competed in the quadruple scull at the 2004 Olympics, finishing in 12th place in Athens.
But his eyes were on the single scull, the event which the great Steve Redgrave admits in his autobiography to not being mentally cut out for and where Great Britain has been starved of Olympic success since 1924, when Jack Beresford took gold in Paris.
Barry was a great sculler himself - although his only Olympic success was a silver medal in the coxless fours in 1964 - and comes from a famous sculling family.
His approach is that of a maverick, which seems to suit Campbell's personality down to the ground.
The idea of training with a saw was inspired by reigning Olympic sculling champion Olaf Tufte, a Norwegian farmer who regularly boasts about the amount of wood he can chop.
Campbell and arch-rival Drysdale are training partners and friends
"Olaf has a farmer's strength - he looks a trees and they fall over rather than having him saw them down," says an admiring Campbell.
But Tufte isn't even the favourite for gold in Beijing. That accolade goes to New Zealand's Mahe Drysdale, a regular visitor to the River Thames, where he picks Barry's brain and trains with Campbell.
"We're friends but it's difficult to be as sociable in Olympic year," says Campbell. "I'll be the first to buy him a pint once we cross the finish line."
Then there is world record-holder Marcel Hacker from Germany and Ondrej Synek of the Czech Republic (who Campbell believes Drysdale will consider his biggest threat).
We've had a specialist programme through the last few months and we're on course to achieve our objectives
Campbell's coach Bill Barry
Since his Christmas on the beach, Campbell has not had things all his own way.
He won the opening World Cup event of this season in Munich in May, beating Hacker, Synek and the mighty Tufte, but subsequently picked up an infection in his right knee, which required minor surgery and lengthy rehabilitation.
"It was a bit of a scary time - it was quite worrying while I was out of the boat, on crutches.
"I wouldn't have wished for it to happen but it has given me the chance to have a rest. I feel fresher and I've had more one-on-one attention that I would have had I been fit."
Campbell has made full use of the Olympic Medical Centre at Northwick Park Hospital and the Institute of Sport facility at Bisham Abbey to get back up to speed.
MEN'S SINGLE SCULLS SCHEDULE
Heats - Sat 9 Aug 0750-0850 BST
Repechages (second chance races) - Mon 11 Aug 0910-0950 BST
Semi-finals - Weds 13 Aug 0850-0910 BST
Final - Sat 16 Aug 0840 BST
Unable to join the rest of Great Britain's heavyweight men's squad on an altitude training camp in Austria he has instead been cycling in an altitude chamber, which improves fitness gains by 25%.
"We've had a specialist programme to take us through the last few months and we're on course to achieve our objectives," says Barry.
Campbell, who wants similar success in London in four years' time, is never going to talk down his own chances.
"To be a successful single sculler you need an ego the size of a football field," he says. "Mine covers two."
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