Roger Springall was raising money for the Motor Neurone Disease (MND) Association, a cause close to him having lost his father to the condition.
He said: “I’m so grateful for all the support I’ve received.”
Having set an initial fundraising target of £1,000, he aimed to complete the miles within a month.
He reached his 70 mile goal on April 21, and had raised more than £1,400 at the time.
After surpassing his initial target, he ran an additional 30 miles, completing a total of 100 miles by May 1.
He said: “As I still had another week or so to go, it seemed the right thing to do was to keep going.”
His endeavours have raised more than £1,500 for his chosen charity so far, with donations still coming in.
He was inspired to take on the challenge by a friend who raised several hundred pounds for a cancer charity last year by walking 30 miles.
Mr Springall often runs the same routes he did as a pupil at Magdalen College School in the 1960’s, and has now signed up for the Oxford Town and Gown 10k race on May 12.
He added: “I’ll be wearing my MND running shirt for the Town and Gown so hope people will still contribute.”