Rugby star Jonathan Davies and his wife Helen will race in the Great North Run to raise money for a cancer unit for teenagers in Cardiff.
Jonathan Davies and his wife Helen will run in the 25th Great North Run
The couple hope to raise as much as they can towards a £300,000 target.
Fundraising for the unit, which would be the first of its kind in Wales, is being masterminded by the Noah's Ark Appeal.
Up to 200 teenagers a year could be treated at the unit to be built at the Children's Hospital for Wales.
The former Wales rugby union and Great Britain rugby league international is urging the public to sponsor him for the 13-mile run on 18 September.
The challenge of completing the race, after a long break from serious running was a tough one, said Jonathan, who retired from playing in 1997.
"The last one I did was the Tenby half-marathon but that was 20 years ago.
"I am training - Helen drags me out for a couple of miles a week," he said.
"We have got four kids, so it is very difficult to get up and leave but we do go up the Taff Trail."
The couple have four children, the three eldest from his marriage to his first wife Karen, who died in 1997 from cancer.
Twelve-year-old Grace and 10-year-old Gena ran the recent Race for Life with Helen.
"It was a very emotional event," said Jonathan, 42.
His involvement in the teenage unit appeal is also important to him as a parent and because the cause is based in Wales.
"I'm amazed Wales was the only country without a designated children's hospital," he said.
Organisers say the new teenage unit will benefit older youngsters who often feel patronised by surroundings designed for younger children.
The couple get fit at the Train Station in Cardiff
As well as sponsoring Jonathan and Helen, the public can also hold their own events to raise money for his specific appeal.
The Noah's Ark Appeal appeal is also part of a wider Welsh Assembly Government challenge to get people in Wales fit.
The assembly's initiative, Health Challenge Wales, is aimed at people taking action against preventable diseases.
There will also be a Welsh mini-version of the Great North Run later this summer in Wales, said appeal organisers.
"I would love to think I can raise more money than anyone else in the Great North Run, that would be a great achievement," said Jonathan.