In a BBC interview from 2012, Prince Harry predicted that his brother Prince William and sister-in-law Kate Middleton would accept the challenge and run the London Marathon the following year, in 2013.
Despite being fully aware that the Duke and Duchess of Cambridge had no plans to participate in the marathon, the Duke of Sussex still made the hilarious promise.

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I believe my brother and his wife will participate in it the following year, which is fantastic, he remarked.
You just said that live on air,” the interviewer Sue Barker said, to which Harry responded, “He’s going to have to do it now, isn’t he?”
Fans erupted in excitement at the thought of Kate and William competing, but a spokesman for the Prince stated: “Harry made a lighthearted, humorous remark. The Duchess and Duke won’t be participating in the London Marathon.”
Clarence House later clarified that the remarks were made “jokingly.”

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In 2013, when a volunteer at the event remarked: “You challenged your brother to run this year,” Harry merely explained it away by citing William’s “age” and Kate’s “pregnancy.”
“I am aware, but what can be done? He is aging and she is pregnant. It is not possible “said he.
Another volunteer spoke for himself and stated: “When Harry was asked to run, he responded, “Oh, here we go… My knee hurts.”
In fact, Prince William said that he would run the marathon in Kenya as well.

Sean Fletcher of Good Morning Britain reported that Prince William had promised to run a marathon in Kenya at some point in their conversation.
Duchess Kate’s response was equally priceless when she was informed: “I’ll believe it when I see it,” she said.
Despite all the talk about the royals participating in the marathon, Kate revealed that it is not an option due to “security,” though Princess Beatrice and her sister Pippa have previously accepted the challenge.