Management consultant Patrick Coffey is taking part in the Blenheim Triathlon in memory of his girlfriend's mother who died from cancer.
Mr Coffey, 24, from Combe near Woodstock, is running his first triathlon to raise money for the new cancer centre in Oxford, which is opening later this year.
He was inspired to compete in this weekend's event, in which competitors run, cycle and swim in the grounds of Blenheim Palace in Woodstock, by Sarah Gibson who died from cancer aged just 51, last month.
Mr Coffey, who works for a firm in St Aldate's, Oxford, said: "My girlfriend Lucia is a cross-country runner and she has raised over £10,000 so far for the new centre.
"I didn't want to sit around while she was doing so much, so for the past couple of months I have been training for the triathlon.
He added: "Sarah, who was a primary school teacher at Marsh Gibbon Primary School, near Bicester, had suffered breast cancer and skin cancer, and in September she was diagnosed with secondary cancer.
"Her bravery, dignity and grace during the whole process was remarkable to see.
"She fought against cancer with incredible strength and determination and tried not to let it affect her family as much as possible. She never once complained.
"One in three people will suffer some form of cancer during their lifetime and the new cancer centre will represent a massive investment in research and care."
Mr Coffey said he had been training hard for the triathlon, swimming in the Thames and cycling 12 miles to and from home to work.
Mr Coffey has been preparing for the triathlon with his colleague Jon Halliwell.
A total of 5,500 competitors are expected to take part in the fourth annual Blenheim triathlon.
The new £109m cancer centre, on the site of the Churchill Hospital, is expected to open at the end of September, and there is a campaign to raise £2m for the centre.
So far, £1m has been raised towards the fundraising total.
The 217-bed centre will combine wards spread across the Churchill and John Radcliffe Hospital and include 10 theatres, a six-bed intensive care and new diagnostic and treatment equipment, including six radiotherapy machines.
Fundraising will help staff invest in equipment and resources not usually paid for by the NHS, including pressure-relieving mattresses, roof gardens and treatment rooms.
To help Mr Coffey's fundraising, visit the website justgiving.com/patrickcoffey
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