Iva Barr, from Bedford, met Charles at the first garden party of the summer at the gloriously sunny Buckingham Palace.
Around 8,000 guests gathered in the back garden of the palace for the soiree on Wednesday, May 15.
Iva was accompanied by her friend and RAF veteran David Green when she greeted the Prince of Wales.
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He said: "Iva just could not believe it. She was so taken back with the fact he spoke to her and was genuinely interested in what she had to say, asking about her running exploits.
"She was her normal self and explained how handsome the soldiers in uniform were."
Ms Barr, one of the oldest marathon runners in Europe when she ran her last race in 2016 aged 88, has run more than 80 marathons.
The regal meeting came about after Iva was invited to the Royal Garden Party for her BEM.
And David called Buckingham Palace weeks before the event to see if Iva could meet the royals.
He said: "On the train down to London with Iva to the Palace I received a phone call from the Palace saying Iva was selected to be presented to the Royal Party.
"We were met at the gates of Buckingham Palace and led to the garden where we were put at the end of the meet line for Prince of Wales.
"He spoke with us for three minutes, which was wonderful."
The party on Wednesday was hosted by Charles who was joined by the Duchess of Cornwall and the Princess Royal.