David Walliams begins Thames swim for charity

David Walliams has set off on his most gruelling task yet – swimming the length of the Thames.

The Little Britain star, who raised £1m when he swam the Channel in 2006, aims to swim 140 miles from Gloucestershire to London over eight days to raise money for Sport Relief.

"I must be a masochist," he said at the riverbank at Lechlade, where hundreds of wellwishers saw him take his first plunge into the freezing water. "I wanted to do something else and I'd just turned 40 and I thought I haven't got much time left because my body is falling apart."

"I thought I'd better get on and do something because one day it's going to be too late."

Thinking of the children who would be helped by the charity provided his inspiration, he added.

He cited a 12-year-old Kenyan boy called Philip who attended a centre which provided him with food, shelter, healthcare and an education, and who wanted to be a pilot.

"He's living in the most desperate circumstances yet he still has great aspirations. I think about him and not wanting to let him down."

Other thoughts would also help to keep him going. "I think about happy things. I think about what I'm going to eat when I get out," he said.

It is unlikely to be the most pleasant of swims in a river that, as TS Eliot pointed out, "sweats oil and tar".

If Walliams successfully avoids the busy river traffic and tourist vessels he will still have to face the perils lurking in the muddy waters, including – among other delights – E coli, salmonella and hepatitis.

Not to mention the 39m cubic metres of raw sewage that finds its way into the Thames every year

Walliams – who has also swum the Strait of Gibraltar, and cycled from John O'Groats to Land's End last year – pointed out that this swim would be 120 miles longer than the Channel crossing.

"That's the scary thing," he said. "It's all right to be full of bravado today but this is day one of what will probably be eight days of swimming.

"So that's what really scares me – the mental challenge of days five, six and seven."

The river water will be bitingly cold, which can cause cramp and involuntary breathing spasms known as a "gasp reflexes".

Walliams – who is expected to burn 4,400 calories a day swimming 17.5 miles – will also have to battle against the notoriously turbulent Thames, known for its unpredictable currents, dangerous undertows and unidentified floating objects.

A GPS tracker linked to the #Thamesswim Twitter hashtag will show online where he is. "The nice thing about this, as opposed to the Channel, is that people can come out and see you," he said.

Despite all the hazard he faces, he said his greatest fear was meeting the birdlife. "When I got here last night there were about 30 swans and I have been attacked by swans in training, so I am actually quite nervous about them," he said.

"When you're in the water and they're coming towards you, fluffing their wings and hissing, it's quite scary."

Walliams – whose swim will be filmed for a documentary to be broadcast before the Sport Relief weekend next March – would not be drawn on what challenges the future may hold.

"The day after I swam the Channel, people said: 'What are you doing next? Do you want to go up Everest?'" he said.

But once he has conquered the Thames, he will be tempted to "hang up my trunks", he said.

"There's swimming the Atlantic but that's too hard. I looked into it and it's 3,000 miles and will take about six months – by which time people will have completely forgotten about you."