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But the granddad took on an opponent about half his age, after he begged his instructor to ask if he could show them what he was made of.
Now after five years of gruelling training Mr Mitchell has achieved the first dan grade, making him the oldest person in Scotland to achieve a black belt with the Japanese Karate Association (JKA).
The Dad-of-three who lives in Haddington, said he had been on "an endless search” to prove himself, with previous activities including mountain climbing and badminton.
His instructor has praised his achievement saying he is “a great fighter and an inspiration”.
Mr Mitchell said: "They were concerned I could have a heart attack or get really beaten up due to my age”.
"When someone attacks you, your instinct is to move back - but if you move in just at the moment they initiate an attack it can be very effective, particularly if you have long levers, which I have.
"So he stepped in, I stepped in and stopped him in his tracks. It happened twice more.
"I think he was getting what they were afraid I would get."
He added: "I had been fighting all my life to prove myself. I didn't know why I was doing it at the time, but now I feel like an inspiration to my family - and I feel relief."
"I would hope the next mountain climb, or badminton competition, would be my masterpiece.”
"But it wasn't until I got my black belt that I realised I had been released from this and that I had achieved that.”
During his grading in October he shocked the instructors, when he stopped his much younger opponent in his tracks.
When the contest was over, Mike had achieved his black belt - which he described as "an extraordinary moment".
When he started training through the karate belts at the age of 69, the former PE teacher said he was intimidated by competing at events full of children with higher grades.
As scores of parents watched from the balcony he said he “felt like running away” and during one grading he considered leaving.
Mr Mitchell was well into his 70s when he started training with brown and black belts.
But he is proud that he kept going and plans to spend the next two years training for his second dan.
Paola Burrows, his instructor at the JKA Bass rock Club in Gullane, said:
"People think that if you go to enough lessons you will become a black belt, but it just doesn't work like that," she said.
"You have to prove you are worthy of a black belt and Mike did more than this at his first dan grading.
"He was up against an opponent much younger than him who was incredibly shocked when Mike stopped him dead several times."