Ross O'Sullivan, 25, from Cardiff, is hoping to complete the epic 354km (220 mile) challenge of the River Severn in 20 days. He hopes to beat Kevin Brady, 32, who took a month to do it last year.
Mr O'Sullivan started at the source near Llanidloes, Powys, on Wednesday.
But he expects to walk the first 80km (50 miles), saying the river is too shallow to swim at this point.
He will then meet his support team at Pool Quay on Saturday when the swim gets under way in earnest.
"My biggest worries are getting hypothermia, the fast-flowing currents or swallowing too much water," he said.
"There is an area just past Gloucester where moving sandbanks create whirlpools and boats have been caught and crashed into underwater tree trunks.
"If I hit one of those, it will be game over, I will be sliced in half."
For the first three days Mr O'Sullivan will be alone without any camping equipment and he is hoping pubs, hotels or people he meets will help him out. Failing that he will sleep underneath the stars.
When he reaches Pool Quay he will meet up with fiancÃ©e, Caitriona Hurley, who is his main land support and two volunteer paddle-boarders who will be his helpers in the water.
He said: "I've been an open water swimmer for about six years and when I moved to Cardiff from Ireland two years ago, I was looking for a new challenge.
"I visited the River Severn and I just thought it looked magic.
"I have been training five days a week and loading up with carbs.
"I have been practicing in the Severn to get my immune system ready but my stomach does feel a bit strange."
He will be raising funds for motor neurone disease sufferers in memory of his uncle Jimmy who died from the disease.
He said thoughts of his uncle will motivate him when times are tough.
"He was a really inspirational guy and what happened to him was horrendous so I want to do all I can in his memory."
Mr O'Sullivan will be updating his blog with his progress.
A swimmer is taking the plunge in his bid to swim Britain's longest river.