“You just don’t think you can do it until you do it,” says Mitch Cook, a Baltimore dad who finished his first marathon this year after losing nearly 60 pounds — an achievement he says fulfilled his New Year’s resolution.

Cook, 30, crossed the finish line of the Baltimore Running Festival in October. He says he resolved to lose weight last December due to multiple health issues, including acid reflux and high cholesterol, and because he wanted to be healthy for his two young sons, who are 3 and 8 months old.

“I was like ‘I’ve got to change’,” says Cook, who weighed 225 pounds at the time.

“Going into 2019, that’s exactly what I did,” Cook says. “My New Year’s resolution was to get down to 185 pounds. It just sounded like a reasonable number.”

Cook, now 164 pounds, started the process of losing weight on January 1. Here’s how he did it.

Cook, a busy salesman who has to reach a sales goal at the end of every month, says he understood it was necessary to have a plan to achieve his weight loss.

“For me, having a New Year’s resolution is great,” Cook tells NBC News BETTER, “but if you don’t know how you’re going to get there — because most of us don’t when we set out on that New Year’s resolution — then how does that end result happen?”

Cook’s plan consisted of three simple changes.

Cook downloaded the weight loss app Lose It! on his phone. He says the app combined data from his then-weight with his weight loss goal and gave him a calorie “budget” each day to reach that goal.

Cook reduced the amount of red meat he ate, replacing it with less-caloric options like ground turkey, and swapped out junk food for high-fiber fruits and veggies.

“I changed everything about how I ate,” he says.

He also started paying attention to portion sizes, which he says he had never done before.

“I’d sit there and eat an entire bag of potato chips or a bag of candy and I didn’t even think about it,” he recalls.

Now, Cook is much more conscious about the amount he eats, and even measures his food on a kitchen scale.

“I read the box, read the bag — whatever the food comes in — and portion it out accordingly and add it into my daily budget,” he says. “It’s easy, and you don’t need that much food to be satisfied or filled up, and if I have a craving for something, I eat a small reasonable amount of it, my craving is gone.”

He says he still occasionally eats sweets he loves, including chocolate.

“I just eat it in better portions,” he says.

Cook, who spends a lot of time on the road as a salesman, says that meal planning was essential to his weight-loss resolution.

“Going from eating out all the time to packing my lunch every single day was a huge difference,” he says.

He also packs healthy pre-planned snacks for when he has cravings between meals — almonds, carrots, bananas, apples and protein bars.

“In my truck, I always have a bag of almonds,” Cook says.

“It’s a good snack, I’m filled up afterwards,” he adds.

Cook was familiar with the idea that “it takes 21 days to form a habit.” He says he knew if he could stick to his new plan for that long that he could transform his routine.

“I just kept telling myself, ‘Do it for three weeks, and it will become part of your daily life,’” he says.

Tracking his progress on the app helped, he says, because the app gave him an estimated timeframe for reaching his goal, which kept him excited about reaching day 21.

Gritting through those first few weeks paid off. After two months, Cook lost 35 pounds just by changing his diet alone.

In March, Cook decided to sign up for his first marathon. It was a major challenge for the dad, who says he never ran a foot race before — not even a 5K.

Cook says he followed the Hal Higdon marathon training program for beginners. He says the program, which had him start off running short distances, gradually acquainted him with longer and longer distances until he was ready for the full 26.2 grueling miles of a marathon

He says long-distance running added more calories to his daily budget, so he could eat more and still maintain his weight loss.

“I still love to eat,” says Cook, who lost 58 pounds by August, two months before he crossed the finish line.

Finishing his first marathon while his wife, Liz, and kids cheered from the sidelines was “pretty incredible, honestly,” says Cook.

“It was a huge bucket list goal for me and it got me completely addicted to running,” he says. “It was an incredible way to cap it off.”

The dad says his resolution for 2020 is to maintain his weight loss and run at least two more marathons.

“Not to sound dramatic, but it is life changing,” Cook says. “I’m a completely different person right now than I was January 1, 2019, that’s for sure.”

Want more tips like these? NBC News BETTER is obsessed with finding easier, healthier and smarter ways to live. Sign up for our newsletter and follow us on Facebook, Twitter and Instagram