British adventurer Dave Cornthwaite  is setting off on a 1,000-mile swim down the Missouri River on Friday.

Cornthwaite is starting in Chamberlain, S.D., and will finish in St. Louis about 50 days later.  He will average about 20 miles of swimming a day.

At that pace, he should reach St Joseph in an estimated 20 to 25 days.

The swim is part of Cornthwaite’s Expedition1000 quest in which he’s trying to complete 25 1,000-mile journeys using non-motorized transport as a way to raise money for charity.

The 32-year-old Cornthwaite says he feels very excited about the swim. He says he’s used to people telling him he’s “mad” and “going to die.” But he says he knows he and his team are well prepared for the adventure.