Mr Goodey said his inspiration was not being able to make it last year to support Mr Smyth at the marathon and his grandfather's saying of "if it's to be, it's up to me".
Mr Goodey, from Hamilton, said he had done a bit of training, though probably not as much as he should have, and had been out running more than walking.
If his grandfather could walk it with 50 per cent lung capacity he could do it too, he said.
Mr Goodey said he would be walking with his step-grandmother Mrs Smyth and a bunch of his grandfather's friends.
They would be walking and talking, and wearing T-shirts with a picture of Mr Smyth.
He said Mrs Smyth had given him a few tips and he would be drinking lots of water and getting a lot of sleep.
"I should be okay."
Mr Goodey said it would be a great experience with some "sweat, tears and a bit of reflection".
Mrs Smyth said she had done a lot of training on her own, as well as some of the longer walks with the walking group at Lake City Athletics.
She said this week, leading up to the marathon, had been "a bit emotional".
Other family members would also be walking the last part of the marathon with her, she said.
Mr Smyth had told her last year that she should carry on with her 22nd this year.
- The Rotorua Marathon takes place this morning, with events starting and finishing in the Government Gardens. Motorists travelling around Lake Rotorua are asked to take extra care.