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Much younger men would be right up for having a six pack and biceps like this OAP, that's for sure.
Millen, who is from Kent, has worked as a personal trainer for 20 years, but he gives most of the credit for his own body work to 'the best gym in the world' - the great outdoors.
When he's not Instagramming or working out, Mike also picks up work as a part-time model.
He does his training by swimming in lakes, running and hiking, and - peculiarly enough - taking a cold shower in his garden every evening.
Whatever he's doing, it's clearly working.
He explained to the Daily Mail: "Training outside brings so many more benefits as we are with nature. It's not only good for our bodies but our souls and mind as well.
"Fitness is so important to me because all my life it's given me a fantastic quality of life and continues to do so every single day.
"My family think I'm strange and worry about some of the things I do but they're still quietly proud of what I am I think."
Mike prefers working out in the wild to being inside a gym - though it seems he does do some stuff in there as well - and uses mainly calisthenics workouts using his own body weight and minimal equipment to keep in shape.
He continued: "The calisthenics training I do I find best done outside in a forest, on a beach, in a park, pretty much anywhere. There are always trees to climb, rocks or logs to use and to play with.
"The coldest place I've trained in was Iceland, on a beach in January when the temperature was around -12C. I actually swam in the sea briefly with the icebergs."
If you want to pick up some tips and train like Mike, he hosts sessions twice every week on LiveNOW's UK based virtual fitness platform.
Speaking about his role, he said: "I really enjoy leading the virtual sessions and sharing my experience and knowledge with a much younger audience.
"I don't do any heavy lifting and on others days I do some cardio. As well as training indoors in the gym or at home leading virtual sessions, these exercise routines can all be done outside and are always fun.'
"It's never too late to start a fitness regime. No matter their age or ability, there is a correct program out there for everyone."
"We all have different levels but whatever program of level you choose, it's going to give you a better quality of life and that's the most important thing.
"I would suggest to start walking. It's free, very little equipment required, just comfortable shoes and you're out in the fresh air with nature.
"Always make fitness a fun thing and find a program that you enjoy."
Featured Image Credit: Mike Miller/LiveNOW Fitness
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