See more pics of David's swim
It's a challenge that would daunt even the most accomplished of athletes.
And swimming the length of the Thames for charity is starting to take its toll on David Walliams.
As well as suffering from the aches and pains associated with such a grueling task, the comedian has contracted a stomach bug.
Speaking to the BBC at the start of day three of his 140-mile challenge, David revealed he was feeling unwell, probably as a result of "taking on the river water".
"So I'm really really feeling sick. And I haven't been able to eat anything…So this is a rough morning," he said.
But the big-hearted star remains upbeat.
"Well I like to stay positive," he laughed when asked about the challenges of the day ahead.
"This is a journey, it's not a race. At some stage say in the next decade I would like to end up in London!
"If I get seriously, seriously ill and have to have a day out the water, so be it. We'll get back in again the next day."
Photo: ę Getty