Amy Sawtell, 26, finished her first marathon in five hours, six minutes and 24 seconds, to raise money for Alzheimer's Research UK.
Her grandfather, Harold Sawtell, who has lived in Ruislip all his life, was diagnosed with the disease eight years ago.
Ms Sawtell told getwestlondon: “The marathon itself was particularly poignant for me as my grandpa spent most of his working life in London as telephone engineer at Selfridges.”
Speaking of the day itself, she said: “It was incredible. I'd never even been to watch before so it was totally overwhelming. The crowd really pull you through.”
Ms Sawtell, who works as a merchandiser for Marks & Spencer, is asking for support from her local community for the "cruel disease".
She added: “There were days when my boyfriend had to drag me out of the house to run. All of the pain of training for the past six months was worth it.”
Mr Sawtell, 87, and his wife Patricia are well known in the Ruislip community. They were involved with the 6th Perivale Scout Group and would help make costumes for their productions.
Ms Sawtell's employer are aiming to match any donations made. To find out more, visit her fundraising page.