The Little Britain comic is preparing to swim the length of England's River Thames next week (beg05Sep11) to raise money for charity.
The funnyman admits his training has been punishing, telling London's ES Magazine, "I was hit by rowing boat oars, attacked by swans and got covered in insect bites from all the bugs that live in the weeds. It was also very, very cold."
But Walliams insists exercise has saved him from going wild, recalling the Little Britain: Live comedy tour which coincided with training for his swim across the English Channel in 2006.
He adds, "Many people in my position - comedians or musicians - might have taken advantage of the situation, going out after each show and getting drunk and high, trying to s**g anything that moved.
"Instead I'd go back to the hotel, have some herbal tea, go to bed, get up, swim for three or four hours, have lunch and do another live show. It put me on an even keel."