The challenge is to swim 2.4 miles, cycle 115 miles and run 26 miles - and Edward Banks is preparing to accept it on behalf of St Luke’s Hospice.
The 30-year-old, from Fulwood, has signed up for the Cleveland Double Steelman Triathlon in Richmond on July 20.
It follows him taking part last year in a half triathlon in North Wales.
“It was very difficult, but I wanted to do the next level,” said Edward, who is about to leave Sheffield for Cheshire to train as a teacher. “I have enjoyed doing the training and I’m looking forward to the day.
“I know it will be difficult, but it will be an achievement that stays with me for the rest of my life.”
Edward, married to Charlotte, with children Evelyn, four, Elijah, two, and Arthur, eight weeks, hopes to raise at least £750 for St Luke’s.
“We have lived in Sheffield for four years and we are moving at the end of July. We wanted to do something for a local charity.”
*St Luke’s golf tournament, at Abbeydale Golf Club, raised more than £5,000. A total of 34 teams of four took part in the event, sponsored by Elevation Recruitment, and organised by the Rotary Club of Sheffield.