He was greeted with huge cheers when he reached the Old Trafford finish line during half-time in the Rugby League World Cup final.
Sinfield has been raising money for motor neurone disease (MND) charities after the diagnosis of ex-Leeds Rhinos teammate Rob Burrow in 2019.
He said he “just wanted to be a great friend” by fundraising to find a cure.
“If we can all try and be a bit of a better friend from time to time, I think we’ll have a bit of a better place to live in.”
The 42-year-old called Burrow “an absolute champion” and praised “the way the Burrow family have been so courageous and brave”.
Speaking during the Australia v Samoa game, an emotional Sinfield thanked the public for their support, saying: “There’s one thing for sure – that our country cares.
“It cares about people who need help and that MND community need us, they need support, they need love and we’ve got to find a cure.”
Sinfield’s task started in Edinburgh on Sunday, and passed through Melrose, Newcastle and York.
The Oldham-born star reached Old Trafford in Manchester on Saturday afternoon after covering 300 miles (482km).
He surpassed his fundraising target of £777,777 – a nod to the number 7 shirt worn by Burrow, who greeted him at Leeds’ Headingley Stadium after the Yorkshire leg on Friday.
The route covered locations that resonated with Burrow, ex-Bradford City footballer Stephen Darby and former Scottish rugby union player Doddie Weir, who all have MND.
The funds will be split between five charities supporting people impacted by the debilitating condition, which affects the brain and nerves.