But their decades-long friendship is now defined by what happened after those sporting highs and its endurance through some of the toughest times of all.

Burrow was diagnosed with motor neurone disease (MND) in December 2019 and since then he and Sinfield have raised nearly £10m for research and support for sufferers.

The image of Sinfield carrying Burrow across the finish line at this year's Leeds Marathon has come to embody the closeness of their relationship. Burrow's wife Lindsey also completed the marathon that day.

Burrow and Sinfield have now written a book, With You Every Step, encapsulating their friendship as lessons for others in a celebration of love, support and connection.

"I wanted to do this book with Kev to highlight the importance of friendship and show that there are many ways to be strong," said Burrow.

"I am hoping the book will appeal to young boys in particular and show that friendship can be based on love and vulnerability."

His voice, complete with Yorkshire accent, is now generated by an eye-gaze computer since MND robbed him of the ability to speak.

He said: "I used to love reading stories to my three young children and I hope this book will give them an insight into what true friendship is."

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'He'll never understand the inspiration he's given me'

Of their friendship, Sinfield, who has just returned from duty with the England coaching staff at the Rugby World Cup, said: "When one of your best mates is given the diagnosis Rob got, then you have a choice whether you stand shoulder to shoulder or not.

"I wanted to help. I think the world of him. What's become clear is the fight and spirit I knew Rob had is shown more than ever.

"He'll never understand the inspiration he's given me and the real perspective he's given me on what is truly important in life."

Burrow, who played for 20 years with Leeds Rhinos, England and Great Britain, added: "Kev was my captain and he still is my captain. He always will be.

"We played together for years. We sat next to each other in the dressing room. We went through the highs and lows of rugby league.

"He always looked after us and here he is still looking after us all."

Seven marathons in seven cities in seven days

Sinfield's next fundraising challenge in aid of MND is to complete seven marathons in seven cities in seven days.

"It is the money that makes the difference. If we're going to find a cure then, as we saw with COVID, you throw enough money at something, you will find a cure."

Some of the proceeds from sales of the book will go to the MND Association and the Leeds Hospital Charity. For Sinfield and Burrow, the message of looking after each other is just as important.

Sinfield added: "That's the great thing about being a human being, if we can help people along the way to have something similar to what we have, then the world's a far better place."