Sinfield, or 'Sir Kev' as he is affectionately known, set out to raise £77,777 for charity in honour of his close friend and former teammate Rob Burrow - Leeds Rhinos' iconic number seven - after he was diagnosed with MND last year aged just 37.
The 40-year-old, who was named runner-up in 2015's BBC Sports Personality of the Year, completed his final 26.2-mile journey this morning just outside his home town of Oldham.
At that stage, the amount raised on his JustGiving page stood at more than £1,130,000. It's still heading upwards quickly, and you can add your own donation here.
Burrow and Sinfield played alongside each other for Leeds Rhinos, winning everything there is to win in Rugby League together, as well as both playing significant roles for Great Britain and England.
While there is no cure for MND, this money will go a long way to helping fund the fight to beat the illness, as well as providing much needed help for those suffering with the disease, including Burrow and his young family.
Speaking after completing the final leg of his marathon earlier on Monday, Sinfield said: "I've run out of different ways of saying it now, but I can't thank people enough for their support this week.
"It's completely energised us on our runs. I was blown away, and it's blown everyone away the way that the community has got behind it."
"There have been so many messages that I've read from people who've been affected and families that have been wiped out by such a cruel disease.
"So, just a huge thank you. It'll make a real difference and let's hope everyone stays fit and healthy."
He continued: "The support has been incredible, but this has been about Rob.
"It's about a group of friends and teammates who had some wonderful times together, and now one of them is in a rough patch and we're coming out to help.
"The big message from me is that sometimes you just need to help a mate, and that's all we're trying to do.
"We're absolutely humbled by the support, the money will make a huge difference. In the simplest terms, we've been on seven runs, that's all we've done."
This isn't the end for Sinfield's efforts to raise money for those suffering with MND, but it's now time for a well-earned rest.
Sinfield concluded: "I'm really looking forward to getting into the ice bath."
Make sure you take a beer in there with you, Kev. Maybe it's time that 'Sir Kev' became more than just a nickname?
Do the right thing, Lizzy.Featured Image Credit: Simon Wilkinson/SWpix.com/Shutterstock