"As much as it did hurt, I felt privileged to feel pain," he said.

The reason it was a privilege to Kennedy was because the person he was doing it for, Jodie Bell, can no longer feel pain after being diagnosed with the debilitating motor neurone disease (MND).

"I did feel like giving up a lot of the time," he said. "But knowing I was doing it for someone who can't feel pain kept me going. I knew it was only pain, it will subside."

The 552 kilometre journey along the Great North Walk from Queens Wharf Brewery to Hunters Hill and back took Kennedy six and a half days. Kennedy said he is the first person to have completed the full trek.

The idea to take on the mammoth challenge came about after Kennedy met Jodie's husband Matt on another trek two years ago. He noticed Matt was wearing a Team Jodie shirt, and learned that Jodie suffered from MND.

"It does make you realise how important time is," he said. "There's no known reason why people get it (MND) and no way to stop it.

"It really gives you an appreciation for life and that everything you can do, you should do while you still can."

So he set out to complete the trek while raising money for MND research.

He completed long distance runs and even went up Mount Tomaree 50 times in one day to prepare. But Kennedy said it was hard to fully prepare as the trip had not been done before in one go.

He left on Sunday, June 16, arriving in Sydney on the Tuesday at 9am then making it back to Newcastle on Saturday night. He said he managed to run the way down, but that "back was more of a hobble".

The hobble was because he suffered either a stress fracture or muscle tear 220 kilometres in.

"I had to correspond with a doctor over the phone," he said. "I had to be careful with what pain relief I could take, but I was having up to 12 Panadol a day."

To make up the time, his days stretched from 15-16 hours to 21 hours of trekking.

But Kennedy said he wouldn't change a thing after seeing Jodie waiting at the finish line in the pouring rain.

"That just made everything worthwhile," he said.

Along with the motivation from Jodie, Kennedy said his support team kept him going, with friends tagging in and out of the trek at various stages.

But would he do it again?

"For a good cause," he said. "A lot of people doubted it could be done, but now I know it can."

Kennedy has so far raised more than $21,000 for FightMND. Donations can be made at support-fightmnd.everydayhero.com/au/the-gnw500-for-fightmnd-team-jodie