After three hours, Callum Johnson turned down a bacon sandwich as he thought he may have bitten off more than he could chew.
The British light-heavyweight spent Tuesday flipping a tyre an overall distance of four miles in order to help raise money for the NHS.
With thousands of lifts, pushes and shoves of the tyre - which he was told weighs around 50kg - he covered four miles in a time of four hours and 49 minutes.
"It was more of a mental struggle than a physical one as I didn't think I'd be out there doing it that long so after three hours I was just thinking 'oh no'," Johnson, 34, told BBC Sport.
"My forearms are in agony and I can barely move my hands as a result but other than that I am OK. I had a pounding headache after it and thought I may be suffering today but I don't feel too bad.
"My closest neighbours are half a mile away and they brought me out drinks. We kept a safe distance and at one point he brought me a bacon sandwich which I just couldn't face.
"The whole thing was flipping bonkers. I think lockdown has sent me mad."
Johnson - who has 18 wins and one loss as a professional - has joined in with fitness challenges set by his friend and Royal Marine Ryan Richardson.
A project which hoped to raise £500 for the NHS in its fight against the coronavirus pandemic has now had more than £21,000 in contributions and has even seen Olympic champion swimmer Adam Peaty get involved in a 100km virtual cycle.
After completing around 1,200 burpees in an hour last week, Johnson was set the tyre challenge by someone on social media and carried out the task up and down a road from his home on the outskirts of Boston in Lincolnshire. He has now vowed to perform a whacky fitness task each Wednesday.
Next week, an adapted half Ironman has been set for him. Rather than swim, bike and run, Johnson will perform 20 minutes on battle ropes in his garden, move on to a 56-mile static bike leg and finish with a row of more than 13 miles.
'My life has never been so normal'
Just weeks before his world-title shot and only career loss against Russia's Artur Beterbiev in 2018, Johnson cried during BBC Sport filming when reflecting on the death of his father in 2016.
Despite having a fight postponed as a result of the coronavirus shutdown, he says life away from the ring - and a break from frequenting the Manchester-based gym where he prepares with trainer Joe Gallagher - has offered him a pause he desperately needed.
"I've gone through some dark times and struggled mentally," Johnson, who won gold for Scotland at the 2010 Commonwealth Games, added.
"If I am not doing things like this with challenges and tasks I can spiral pretty quickly to a dark place. It's a medication of sorts.
"I think this lockdown has improved me massively. Since dad died I would say I have had a little bit of depression for probably the full four years.
"But this four weeks I have lived in such a routine it's changed things for me. It's a horrible thing that has happened and I wish it hadn't of course but it's kind of woken me up and given the nudge I needed.
"People talk about when 'normal life' will return. In many ways as a boxer my life has never really been normal before this. Now I am at home eating with family, taking the kids out. My life has never been so normal."