Youths at Christchurch Mens' Prison are running a marathon for the first time today.

Photo: File photo/Department of Corrections

Twenty-five men, all under the age of 20, began running laps behind bars with some of the guards just as the sun was coming up this morning.

Principal corrections officer Gary Smallridge arrived at work just before 5:30am and there were lights everywhere, half an hour before usual, he said.

"They're all up, half of them and some of them have had breakfast at 3am like they'd been trained up to do."

They had received advice from an expert who came in to talk to them about running their first marathon, he said.

"A lot of them are really quite into it and have done all that they've been asked to do," he said.

"They're looking fresh as daisies."

He said it all began when the Duke of Edinburgh Hillary Awards were introduced in 2016 and the men had to do some fitness.

"That's what kicked it off to get them really working," he said.

"They come to prison and half of them had been sleeping in until lunchtime and staying up all night.

So we're just trying to ... give them some employability skills," Mr Smallridge said.

"Getting them up and thinking about the day ahead rather than sleeping half way through it."

He said alongside those battling the marathon today, 13 were attempting a half marathon and others who were bigger or had done little exercise were tackling a 10km walk.