Marathon runner David Davies has pledged to raise £2,000 for a hospital.
Mr Davies, 35, from Ambrosden, near Bicester, has entered the London Marathon in April for the first time.
He is being sponsored for the Great Ormond Street Children's Hospital, London, where a nephew of his wife Christine has had a long series of operations.
He said: "The relative, Richard Coles, who lives in Cambridgeshire, was born with a skull deformity and had operations at the hospital once a year.
"As the hospital looked after him so well, I decided to raise funds. Richard is now 18 and does not need any more operations. He can lead a normal life and has become an apprentice mechanic for a car company."
Mr Davies is a custody officer at Campsfield House Immigration Centre, in Kidlington. A group of staff have formed a running club and go for training runs after work.
Mr Davies has completed two half-marathons, the Great North Run in Newcastle and the Great South Run in Portsmouth.
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