Joe Wicks is going to exercise for 24 hours straight to raise funds for Children In Need - and fears he could end up bursting into tears.
The Body Coach, who kept the nation fit when lockdown first began with his daily PE lessons, is hoping to raise millions by taking on the "savage" challenge.
The 35-year-old was today unveiled as this year's candidate for the Radio 2 BBC Children In Need Challenge.
The dad-of-two told the PA news agency: "It’s going to be tough, I’m not going to sleep, there’s no end to it. I’ve just got to keep going.
"It will be draining and I know that when you get true fatigue and you’re really exhausted, you get really emotional.
"I’ll probably end up, at some point, bursting into tears."
He added: "But hopefully people will be there to pick me up and say, 'Come on, keep going'."
Joe, who was recently awarded an MBE, also chatted about the challenge on today's Zoe Ball Breakfast Show, telling her he was looking forward to eating the 10,000 calories a doctor has estimated he'll need to consume throughout the day in order to keep going.
He told Zoe: "This will be a really tough challenge, I've done endurance things, I've done long runs and marathons but this, psychologically, when you get to that 3, 4am mark and you're so tired and have to go again!
"But I'll tell you what I am looking forward to - all the food! The doctor said I need to eat 10,000 calories over 24 hours, which I'm going to try."
Zoe said she fears Joe will be so "tired and delirious" that he'll "forget how to eat", but she has every confidence that he can make it through the challenge.
"But I’m sure we’ll make some high calorie drinks, and get some avocado and peanut butter in there and fatten it up.
"Hopefully, I’ll have predominantly healthy food but with the odd chocolate bar or packet of fruit gums.
"I want to keep my blood sugar levels stable rather than just smashing the sweets and the chocolate."
Last year, Rylan Clark-Neal took part in a 24-hour karaoke challenge for the charity, with surprise guests joining him.
But celebrity guests will only be able to join Wicks remotely because of the pandemic.
The PE guru, who is Children In Need’s schools ambassador, will be allowed a five-minute toilet break every hour.
* The Joe Wicks 24 Hour PE Challenge will see Joe start his work out at 9.25am on Thursday, November 12, endeavouring to finish a day later at 9.25am on Friday, November 13