The annual Sydney Morning Herald Sun Run saw more than 6500 runners participate to raise money for their selected charity in 2016.

This year Oli, who is excited to start year 3 shortly, will join the runners. He told his mother Annie White he was running "in memory of his grandmother".

"I said that a lot of people do a fun run or a sports event in memory of somebody, and he said he wanted to do it in memory of his gran," Ms White said.

Oli's grandmother died three years ago from cancer, when Oli was five. Ms White said the two were very close and the time was particularly hard on him.

"She was living in Spain and Oli was born here," Ms White said. "She used to write lots and lots of letters. She was a very special lady and both of my children had a strong relationship with her."

A few months ago, Oli told his mother he wanted to run a marathon. This came as a surprise to Ms White.

"Nobody in our family has done a marathon before. He must have seen it when we were watching the Olympics," she said. "I don't know where it came from."

Since signing up, Oli and his mum train regularly to prepare for the run.

"We try to go out early in the morning before it gets too hot and it's good to do that during the school holidays," Ms White said. "It's been a lot of fun."

Although this will be his first big run, Oli is no stranger to sports. His mum said Oli has been "kicking a ball around since he could stand up".

"He's always loved sports and he loves watching sports."

Oli will be cheered on by family as lines up at the start with hundreds of other runners, keeping his grandmother in mind the entire time.