David Harbour has developed a new relationship with exercise thanks to his acting career.
The 47-year-old talked to PEOPLE about "being a chameleon" and how playing different roles helped him to jumpstart his fitness.
Harbour says he was a "really nerdy kid" growing up and never played sports, admitting that he had resentment for the athletic boys his age and "resisted athleticism" for most of his life.
Now, he tries to do something physical every day and he has partnered with Brooks Running to encourage others to work out in a way that works for them.
"In my 40s, I started to realize that there's things I want to do and my body was not going to sustain if I kept on the path I was keeping on," he tells PEOPLE. "I just really thought I had gone over the hill. And it was like, on a downward trajectory from there."
"There was a moment where I was crossing the street in New York and there was a car coming and I thought, 'Oh, I gotta sprint a little bit across the street,' and I just couldn't sprint. And I was like, 'Oh I guess that's gone. I guess I'll just never sort of run across the street.' I mean at this point I was like 270 lbs. and my knees were kind of shot. And I just sort of thought that that's the way life was, that was the trajectory of your body," he continues.
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It wasn't until the past few years that the actor "developed a new relationship" with his body when he had to drop 80 lbs. for his role as Sheriff Jim Hopper in Netflix's Stranger Things. He says his weight loss and the new experience with fitness was eye-opening.
"It opened up a whole new world, [I'm] a lot more pliable and a lot more teachable, even in my mid 40s, than I ever imagined it could be. And it felt like a rebirth," Harbour says. "It was really refreshing and really exciting to feel like after months of training like, Oh now I can just sprint across the street if a car comes. Just simple little things like that that were really exciting for me."
To prepare for Stranger Things' latest season, Harbour went through eight months of training, sticking to an exercise plan and beginning intermittent fasting. He says he goes back and forth "when I'm in my big dad bod mode," and enjoys the body transformations that he's able to make throughout his career.
"I love my big body as much as I love my lean athletic body," he says. "Something about being an actor is you are allowed to live in different skin and I like being a chameleon in that way. I much prefer to be more of a chameleon, and figure out how it feels to be in different skin."
But Harbour says he doesn't always love the process.
"When you're training for something, you get experts around, trainers and nutritionists, and that's really helpful, but also it gets tiring," Harbour explains. "I am an artistic, creative personality who, in general, doesn't love to be told what to do. I like to forge my own path. And so it gets a little tiring when you have all these people sort of telling you what to do and so you kind of want to go off and just do your own thing."
"When I'm not training specifically for something, exercise in general can be much more playful, doesn't have to be as grindy or as serious or as right-and-wrong as some of the experts will tell you," he adds. "But then of course when I'm doing too much of that I long for that expertise and the intensity. So it's a bit of the grass is always greener."
When Harbour is looking to "do his own thing," he likes a couple days of weight training and tries to get in four or five days of steady, low-intensity running, which he says is more about managing his anxiety than cardio. He says he keeps his pace at a low rate, or 65-75% of his heart rate.
Being able to run on his own terms is why Harbour partnered with Brooks Running for the brand's new "It's Your Run" campaign, which celebrates all types of runners and encourages people to run in whichever creative way suits them.
"I really started [running] a lot more during the pandemic because I was so stressed out, especially going back to work. I had a lot of anxiety and I went to a PT who told me, 'Look, all you gotta do is run for 45 minutes to an hour, really slowly, and your breathing and your heart rate will level out,'" Harbour explains. "I did that for a couple weeks and after about the second week, I noticed a huge difference mentally and in my anxiety. It all sort of melted away. So it really is more about anxiety than cardio for me, although there are cardio benefits. It really is a mental, spiritual gain."
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"I really liked that because, you know, a lot of my runs are half walking. So according to them, those are worthy of celebration," Harbour tells PEOPLE. "To have a campaign that really fit me and my style of how I approach running, I just thought that was a very sort of welcoming, nice way to look at fitness in a world of all these experts, in a world where I'm constantly being barraged about how to do an exercise and how I'm doing it wrong, to have this attitude of like, it's a playful thing and it's your run and you can go out and do it any way you want to do it. It was very refreshing to me."
Harbour is learning to find a balance between fitness on his own and fitness for work. He says his weight has fluctuated dramatically in the past few years, sharing that he lost about 80 lbs. in between his roles for Black Widow and Stranger Things and then gained the weight back for his upcoming role in Violent Night, which he's currently trying to lose again.
While the actor hasn't "settled" on where he feels most comfortable in his health and fitness journey, Harbour says for now, he's feeling "pretty good" with being able to do something physical each day while looking back on how far he's come.