DAVID WALLIAMS is on his way to London via the Thames, after slipping into the river at its source in Lechlade, Gloucestershire.
In eight days time, with 140 miles under his belt, the Little Britain star plans to be in London.
"That's what really scares me – the mental challenge of days five, six and seven," said Walliams just before the starting the Sport Relief charity swim today.
"I must be a masochist.
"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."
In 2006, Walliams swam the English Channel, but feels the Thames effort is better in at least one respect.
"The nice thing about this, as opposed to the Channel, is that people can come out and see you," he said.
You can sponsor Walliams at the Sports Relief site, where you can also find a map that tracks his watery whereabouts.
There's also a dedicated Twitter page.