Rugby league legend Kevin Sinfield has so far raised more than £300,000 in his attempt to run 101 miles in 24 hours in aid of motor neurone disease research.
He set off from Welford Road, where he is a coach for Leicester Tigers.
The route will end at the Headingley home of rugby league club Leeds Rhinos, where he played for 18 years.
His ex-Rhinos team-mate Rob Burrow was diagnosed with MND in 2019 and Sinfield raised £2.7m last year by running seven marathons in seven days.
The 41-year-old set an initial target of £100,000 for the Extra Mile Challenge, but pledges of support meant he had already exceeded that total before starting the run at 08:40 GMT on Monday.
His route is split into 7km sections and he must complete each one within 60 minutes.
Speaking to BBC Radio Leeds last week, Sinfield said he hoped the experience would be “horrible and unpleasant”.
He continued: “A big part of the challenge is mental. There’s physical parts of it that are going to hurt and be horrible, but I think mentally every hour some of it is going to be torturous.”
Sinfield – who ran the London Marathon last month – reached Nottingham on schedule by 14:40 GMT.
“I’m alright. I’ve been buoyed by the support, it’s been incredible and the money we’re raising is fantastic. I’ve not really gone into that dark zone yet, I think that’s still to come, and I’m looking forward to the battle ahead,” he told BBC Radio Nottingham.
“I don’t think we’re going to have this sort of crowd at 2am. I know there’s going to be a scrap at some stage, but we’re ready for it.”
As he pushed on further north, Sinfield – who was awarded an OBE in the Queen’s Birthday Honours for fundraising and services to rugby league in June – was joined on one section by former Manchester City, Liverpool and England footballer Gemma Bonner.
“I’ve known Kev since we were seven or eight in school. I knew Rob too from that age so I have a long-standing connection with them. They were unbelievable role models for me,” she said.
“I’m grateful to be a tiny help to raise awareness.”
The run will continue throughout the night, with the final leg from Hyde Park in Leeds to Headingley beginning at 07:40 GMT on Tuesday.
Burrow hopes to be at the finishing line but Sinfield said: “Whether Rob is going to be good enough to be there on Tuesday, we’ll have to wait and see.
“If he is, I’ll be absolutely made up. If he’s not, I’ll understand. I’m sure there’ll be a couple of text messages en route where he’s laughing and taking the mickey out of us.
“But I absolutely know he will be with us every step of the way.”
MND affects the brain and nerves and eventually stops muscles functioning.
The government has pledged to put at least £50m into MND research over the next five years.
