Ray Matthews, 75, runs 75 marathons in 75 days
Ray Matthews, from Maltby, near Rotherham, South Yorkshire, completed his challenge earlier after starting out on 2 July.
He said: "I'm feeling better than I should be feeling."
The challenge has raised about £13,000 so far for Rotherham's Newman School, which caters for a range of special educational needs and disabilities.
Mr Matthews ran into the grounds of the school to complete his final marathon and was greeted by supporters.
He said: "It has been tough at times to do 75 marathons in 75 days, but I've done what I said I'd do."
Mr Matthews started the challenge shortly after his 75th birthday and described the marathon series as a 75-day "birthday present" to himself.
He said: "It's been tough at times, but generally better than I had expected.
"I've had so many great people I've never met before who've given me fantastic comments and money as well. I've really enjoyed it.
"Other people have doubted me, but that's fine. I've always had faith that it could be done - that's why I set the challenge."
He added: "Every marathon has been painful, but it has been a pain that has been able to be managed in general."
The majority of the routes he tackled were organised by running clubs in South Yorkshire, but he also completed two marathons in Rotherham's twin town of St-Quentin in France.
Four years ago, at the age of 71, Mr Matthews also ran 150 miles in 36 hours.
He aims to raise enough money to improve the external areas of the school and provide new equipment.