Kent, UK- Fit grandpa Mike Millen is 67 years old and sports an impressive six-pack. If you assumed, though, that this Brit's lifestyle is purely consists of him torturing himself at the gym, you'd be wrong.

Mike Millen has a figure so trim that it makes men less than half his age go green with envy. How does he manage to stay so fit and muscular at such a distinguished age, though, how is it possible?

The certified personal trainer works out six times a week, but he doesn't pump iron cooped up in the gym. Instead, his workouts are all natural.

Instead of hitting the gym, Millen chooses to run through forests and climb over tree trunks. Millen told Femail, a section of the Daily Mail that focuses primarily on issues pertaining to women, that the great outdoors is "the best gym in the world."

Apparently it's not only good for his body, but also for his mental health. "Training outside brings so many more benefits as we are with nature," he said. "It’s not only good for our bodies but our souls and mind as well."

Millen, who also works as a model, is an avid swimmer. In a particularly impressive feat, this 67-year-old once went for a splash in Icelandic January, when the temperature was 10 degrees Fahrenheit on the beach and icebergs were floating around in the water. This guy is really tough!

Mike says his joy motivates him. For him, the most important thing is to just keep moving. Even a walk a day counts. In fact, his enthusiasm for sports began at an early age, according to an interview he gave with Caveman Training:

"I’ve always been involved in sport all my life, some of my first memories are of kicking a football around in the street. I always wanted to be a pro footballer... I made it to A-level, and always loved the training as well as playing."

He played soccer until he was 30 and then tried running marathons: "The marathons made me really fit, but with little strength, so I gradually went away from the marathons."

"For some reason, I wasn’t into heavy lifting, but loved the body weight training as it was so varied and could be done almost anywhere, including using what nature has to offer and I still to this day find being outside the best and most enjoyable way to train."

The 67-year-old clearly loves working out and has a tip for those who are thinking about getting fit: "There is a training program out there whatever your age or abilities are!"

If you need more inspiration, you can check out Mike Millen's Instagram page, where he regularly shares exercises and clips from his everyday life with his 15,000 followers.