Headingley Stadium on a cold, crisp December morning is usually a quiet place. The rugby league and cricket clubs are hibernating, but on this particular Friday there is a buzz in the chilly Leeds air.

Kevin Sinfield has already raised more than £8m to support people affected by motor neurone disease, a group that includes his former Leeds Rhinos teammate and close friend Rob Burrow. The bond shared between them has captivated the nation and helped so many and while Sinfield is a rugby league legend, the former Leeds captain’s reach extends far beyond the sport where he and Burrow made their names.

Sinfield, who now earns his living as defence coach of the England rugby union team, has already completed some extraordinary fundraising challenges, including seven ultramarathons in seven days last year and a 101-mile run in 24 hours from Leicester to Leeds.

This time, it is seven ultramarathons in seven days in seven cities across the UK and Ireland. The opening leg left Leeds on Friday morning, heading to York, and hundreds were at Headingley to cheer Sinfield off, including Burrow. He would not have seen crowds of this size waving him off to major cup finals as a Leeds player, but this clearly means so much more than any sporting occasion the pair have been involved in.

Related: Kevin Sinfield: ‘The longer this marathon takes, the more time I get to spend with Rob’

I was fortunate to be one of a select group to be invited to run part of the opening day of Sinfield’s latest challenge. Every leg of his ultras will include an Extra Mile, to allow members of the MND community to join Sinfield and the runners.

It is a moment designed to symbolise the bond between Sinfield and Burrow and how they are both prepared to go the extra mile for each other. It is a poignant and touching mile, too. Some of the participants involved in York had lost husbands, wives and other relatives to MND. Others have the condition and ran, walked or used their wheelchairs to complete the mile, an extraordinary feat in itself.

Many in attendance speak of how Sinfield’s determination to do whatever it takes to help his friend has inspired those with MND to set their own challenges. One man used his off-road chair to scale Snowdon last year.

The Liverpool footballer Gemma Bonner and the boxer Josh Warrington, both from Leeds, are also present. Sinfield arrives at about 3.20pm; he had been running for well over three hours by then but still has a smile on his face. “Who’s cold and who’s tired?” he shouts before raising his hand with a grin.

However, within seconds he is on the move again, with those invited to run the Extra Mile following him and his crew, which includes his former Leeds teammate Jamie Peacock, around York racecourse. By the time 2023 finishes your reporter will have run about 2,000 miles in the year, taken part in countless races and clocked up personal achievements, but not one single mile has been filled with as much importance and poignancy as this one.

Sinfield’s determination to help Burrow and those with MND has always been obvious by the magnitude of his challenges. They still need about another £2m to help make Burrow’s dream of a specialist MND centre in Leeds a reality, and hope this challenge will raise £777,777, a nod to Burrow’s shirt number while playing at Leeds. But running alongside Sinfield, even briefly, his passion to make a difference and help people almost immediately shines through.

While running together, he tells me every location in the seven days has a purpose. The second leg in Cardiff is close to Gloucester, where Ed Slater, who was diagnosed with MND last year, played rugby union most recently. Edinburgh will raise awareness for the Doddie Weir Foundation. Dublin, he says, will help raise awareness of the condition in Ireland. He will also visit Birmingham, Brighton and London.

The Extra Mile finishes at the other side of the racecourse, but some of us press on to the finish at York Minster, 45km on from the start at Headingley. The roads are flooded with onlookers. Sinfield is emotional when he speaks.

“We’re coming up to Christmas and within this is the big message of how important friendship is,” he says. “Let’s look after each other and make this world a better place.”

In that moment, the overarching theme of why Sinfield is doing this becomes apparent once again: friendship, and looking after those who need help.

Later in the evening I catch up with a friend who, by his own admission, knows nothing of rugby league but knows all about Sinfield’s heroics. “Everyone needs a friend like Kev in their life,” he says. How true that is.