The Leeds Rhinos legend, and current Director of Rugby for Leeds, took on the challenge in early December in support of his friend and former Rhinos teammate Rob Burrow, who was diagnosed with Motor Neurone Disease (MND) in 2019.
He raised an incredible £2.5million for the MND Association and rugby fans are now calling on him to be given a knighthood.
His valiant efforts motivated people across the country to begin similar challenges - including seven-year-old Henry Varley.
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Little Henry, from Yeadon, decided to run one mile every day for seven days after seeing Sinfield on television.
He chose to raise money for a local charity, Aireborough Voluntary Services to the Elderly (AVSED), who help older people in the community.
His mum Lou Varley, 36, said: "The reason he took on the challenge is because he really misses his grandma as she is shielding.
"He wanted to do something nice to help older people who might be feeling lonely at Christmas.
"He initially wanted to do something where he could go and see them but he was scared they would want to hug him and he'd get them into trouble.
"We are big Leeds Rhinos fans so we were amazed at what Kevin did for Rob Burrow and the MND association.
"When Henry saw Kevin's marathon on the TV, he stopped and pointed and said: 'right, well I'll do that."
Henry, who goes to Yeadon Westfield Junior School, started his challenge on Wednesday, December 9.
He went on a few runs with his mum, a couple of runs with his dad, Phil, and a couple with his older brother Ray.
Henry completed some of his runs in Nunroyd Park - where the charity is based - and took their 'Elf on the Shelf' Christmas mascot along with him for his last run.
He finished his challenge on Wednesday, December 16.
In total, including gift aid, Henry has raised more than £700 for the charity.
The money will be put towards Christmas hampers for older people in the area - and Henry will be helping the AVSED team to deliver the special gifts.
Michelle Phillips, Volunteer Engagement Officer at AVSED said: "We could not be more appreciative that Henry chose us out of all the good causes and charities out there.
"For a boy of such a young age to have such a grown up attitude to helping others really shows you that even through the darkest of years, there are rays of sunshine breaking through.
"We have been working so hard to keep the morale of our members up this year, and when Henry turns up on some of their doorsteps with Christmas hampers next week, I’m sure it will help make their Christmases special.
"I do hope that this can make its way to Leeds Rhinos and Kevin Sinfield too and if there is any way they could help us to show our gratitude for his efforts, we would really appreciate it.
"Henry is truly an inspiring little lad and we are so grateful for his and his families support."
Henry has since been given an award and medal by the charity, and has also had his photograph taken to appear in the school newsletter.
Lou, who is also mum to Ray and Coby, said: "The response has been lovely and we have had some really nice messages and quite a lot of donations.
"We were a bit shocked that he had raised so much.
"He took his certificates in and his teacher and all classmates were telling him how proud they were.
"He asked me to share a message on Facebook for him which said: "Thank you. It's the nicest thing you have ever and done and it is nice to donate."
"We are both just really proud of him for going out of his way to help other people."
Henry's Virgin Money Giving page can be visited here.