Most people who swim in B.C.’s Lake Okanagan don’t expect to be in the water for 40 hours – but this is exactly what an American endurance athlete attempting to set a Guinness World Record is aiming to do.

Adam Ellenstein, 39, is planning to swim 105 kilometres from the northernmost tip of Lake Okanagan near Vernon, to Penticton at the south end – all in 40 hours. He began his epic journey early Monday morning, his crew at his side.

“It’s going to be less about my physical capability and more about my mental state,” Ellenstein told CTV News before he began the swim. “And I have the best crew I could hope to assemble.”

The swim is also serving as a fundraiser for Parkinson’s disease, inspired by his aunt, Susan Scarlett, who was recently diagnosed. She will be joining him in the water for sections of the swim.

“The swim has a greater meaning than simply going out to test my endurance limits,” Ellenstein wrote on the swim’s Facebook page, VictorySwim105. "I'm swimming to support and honor my aunt, who has Parkinson's disease. To her own amazement, my aunt is training to swim part of the lake with me.”

Every second of the journey needs to be recorded in order for Ellenstein to be eligible for setting a Guinness World record, so his team is equipped with 10 active cameras and three spares.

His crew is also well-stocked with food: Ellenstein needs to consume at least 10,000 calories each day to keep with up his physical output.

Ellenstein is no stranger to endurance feats: he has completed Ironman and UltraMan triathlons, and even swam 30 kilometres across Lake Winnipeg. His current venture is more than triple that distance.

“There will be hours when I have my head down and I'm really in a zone swimming and am not really present,” he said. “And other times when I really rely on my crew to keep me going.”

Crew member Darwin Holt agrees.

“He’s in a good place,” said Holt. “He’s feeling good, and his job now is to maintain.”

If all goes according to plan, Ellenstein will step out of the lake Tuesday evening around 9 p.m.

Anyone interested in donating to Parkinson's disease, click here.

With files from CTV Vancouver’s Kent Molgat