Comedian David Walliams has passed the halfway point in his bid to swim 140 miles of the River Thames in eight days for Sport Relief.
The Little Britain star started his challenge on Monday but has struggled since Tuesday with a bacteria bug which has left him well behind schedule.
Walliams told the One Show: "I might not finish on time but I will finish.
"Maybe not by Monday but by some point in the next decade, I will finish. You have to work through a lot of pain."
The marathon challenge, which is part of the BBC's Big Splash initiative to inspire people to swim, would normally require Walliams to take in 8,000 calories a day but he has been unable to keep down food.
He is due to reach Big Ben on Monday but Walliams admitted: "It's really hard to keep your head together because as soon as you start going, 'It's cold, I'm tired and I want to get out', you're letting your mind rule everything."
Walliams had raised £438,000 by 0700 BST on Friday as he re-entered the water at Caversham Lock for another day of battling the powerful and unpredictable currents and undertows.
As well as navigating the busy river traffic, the comedian will have to deal with murky water that harbours a cocktail of bacteria such as E.coli, salmonella and hepatitis.
Walliams will also have to avoid the 39 million cubic metres of raw sewage that finds its way into the Thames every year after heavy rainfall.
The distance is equivalent to swimming the Channel seven times. Walliams swam the Channel successfully for Sport Relief in 2006.
He has also swum the Straits of Gibraltar and last year cycled from John O'Groats to Lands End, also for Sport Relief.
To support Walliams visit his pages on the Sport Relief website.
To learn how to swim, or how to improve technique, visit the Big Splash website.