So, David Walliams, what does it feel like to swim the Thames? 'Tiring...'

David Walliams greets his wife, Lara Stone, yesterday after swimming the length of the Thames for Sport Relief (Getty)

He swam for 140 miles, burning 70,000 calories and lifting his arms for 110,000 strokes after plopping into his Thames at its source in Gloucestershire just over a week ago. His assessment yesterday, after completing his epic challenge: "I feel quite tired, I'll be honest with you."

David Walliams dragged his weary limbs from the water opposite Westminster shortly before 6.30pm to cheers from crowds lining the river banks, some of whom had helped him raise more than £1m for Sport Relief. When asked on live evening television news what the most difficult part was, he replied: "Feeling like my arse was going to explode for two days."

"It wasn't really fun. It was 140 miles in cold murky water. The fun bit is when you get out, but it doesn't compare to Alton Towers," he told The Independent.

"Thames tummy" has been an almost constant companion as Walliams swam through increasingly polluted waters. But the comedian also had to endure the psychological ordeal of being alone with his own thoughts for such long periods of time. "I was in the water 11 to 12 hours a day. You have delusional paranoid thoughts. I kept thinking someone was going to drop a brick on me from a bridge," he said.

After a quiet start at Lechlade in Gloucestershire last Monday, Walliams' feat gained a dedicated following as bystanders cheered a growing flotilla of boats and kayaks that have surrounded the tiny silver dome of his swimming-capped head.

The final stretch, down the tidal portion of the Thames through central London, was supposed to be the hardest part of all. In fact the outgoing tide propelled him along at a speedy 5mph.

"I never expected this to catch the public's imagination like it has," Walliams said, during a lunch stop at Kew as he waited for the tide to change before beginning the final stretch through choppy, sewage-filled waters.

"He's the sort of nation's sweetheart," said his mother, Kathleen. "People think they're part of this because they can actually see him doing it. He's rescuing dogs and waving to people when all he wants is to lay down almost. I'm very, very proud of him." At Cookham Lock in Berkshire on Saturday, Walliams helped Vinny, an over-enthusiastic labrador, from the river. Vinny suffers from a bad hip, and couldn't make it out unaided.

Almost 200 children – the entire upper school of Kew Green Prep School – lined the banks as he swam past yesterday morning. A cursory wave and they all went a little bit nuts.

"I just think it's so inspiring," said Caroline Sutton, who watched from the bank with her four-year-old-son, Max. "When you see him coming you almost well up. I don't know how he's doing it."

Comedians Jimmy Carr, Miranda Hart and Rob Brydon have all been out to support him as he made his epic journey, as has former talk show host Sir Michael Parkinson. Yesterday the support boat containing Walliams' wife, Lara Stone, ran aground in a muddy section of the river as the tide began to change direction.

As Walliams plopped into the water at Teddington Lock at 6.45am yesterday, he looked exhausted. "Every part of my body aches," he said in an interview with Radio 1 DJ Chris Moyles.

"To be honest with you, I'm kind of over swimming now and I need to just have a nice lie-down."