A father and son from Felsted took on the Iron Man Triathlon in Spain in memory of murdered family friend Grace O'Malley-Kumar.
Grace, a promising medical student, was tragically killed in Nottingham in June last year at the age of 19.
Her former neighbours, the Knight family, were devastated by her death and have been raising money for The Grace O'Malley-Kumar Foundation, which was set up in her honour.
James Knight, a 20-year-old former Felsted school pupil, took on the Iron Man with his dad Stuart after a year of training.
His mum Helen said: "James found the training really gruelling - he'd never done anything like that before and my husband has.
"Grace would have been so proud of him, which is what kept him going."
The triathlon took place on Sunday, October 28, and saw 2,000 competitors take part in a swim, a cycle ride and a run.
Helen was able to follow both James and Stuart's progress on an app, and saw James as he finished the cycle and started the running section.
She said: "James was sprightly and full of beans. He didn’t have a sweat on.
"James completed the Iron Man in seven hours 11 minutes – an hour and 15 minutes better than he thought. He smashed it.
"It was amazing and very emotional. As James crossed the finish line he cried and I cried.
"He had Grace’s initials on his clothing and as he finished he looked up at the sky and pointed."
Stuart, who walked during the running section because of a hamstring injury, completed the challenge wearing a Grace O'Malley-Kumar Foundation running vest.
The pair are still accepting donations at https://www.justgiving.com/page/jamesknightfundraiser, and have so far raised around £21,000 for the foundation.
James's younger brother Charlie, who is 15, previously ran a book stall at Felsted School to raise money in Grace's memory.
Their middle brother, 18-year-old Ollie - who is in Sixth Form at Felsted - also plans to fundraise for the foundation.
The family has raised around £25,000 between them in total.
Helen said: "I am very, very proud.
"We might all do Iron Man in a relay. Between us all I might do a swim, Ollie might cycle and Grace's mum Sinead might do the run."
Stuart and James also plan to tackle the Iron Man Triathlon again next year in Sweden.
Grace played cricket for Essex, and also played hockey alongside many Felsted pupils.
Her family used to live opposite the Knights, and the two families regularly went on holiday together.
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Helen said: "This has had a massive impact on us as a family.
"You always read about it happening and think it’s never going to happen to someone you know. It’s catastrophic.
"The ripple effect is vast on all the children who knew Grace. She knew so many kids who knew her and looked up to her.
"She was an amazing human being. She was beautiful inside and out, and she was funny and lit up the room."
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