Mark Bingham, who works at Wards Garden Centre, took part in the 26.2-mile race on Sunday to raise funds for Brain Tumour Research.
His wife Fiona, a 33-year-old career adviser, was diagnosed with a grade three astrocytoma in 2015.
Her diagnosis came after a visit to Boots for prescription sunglasses when opticians discovered pressure behind Fiona's eye.
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Following surgery, chemotherapy and radiotherapy, Fiona sadly died last year.
Mark said: "I am absolutely knackered but I couldn’t be happier that I've completed the marathon.
"Fiona would say I’m crazy for doing it but I’m sure she’d be proud that I’m raising money for research into brain tumours.
"So little is known about brain tumours which is why research funding is so crucial.
"I'm hoping to raise at least Â£3,500 for scientists to continue trying to find a cure."
Michael Thelwall, head of community fundraising for Brain Tumour Research, said: "Mark's determination and commitment are fantastic and I hope his story will provide inspiration to others whose lives have been affected by a brain tumour.
"Brain tumours kill more children and adults under the age of 40 than any other cancer - and they kill more women under 35 than breast cancer - yet just one per cent of the national spend on cancer research has been allocated to this devastating disease.
"For too long, brain tumours have been a neglected cancer.
"Experiences like Fiona's remind us all that we cannot allow this desperate situation to continue.
"We are extremely grateful to Mark and offer our congratulations to everyone who took part in this year's event to raise money for charity."
To make a donation to Brain Tumour Research via Mark's JustGiving page, visit www.justgiving.com/fundraising/mark-bingham5