Stefureac, 52, has since run at least one a marathon on all 7 continents -- and ran at the North Pole this past August

Derek Stefureac was having a normal day at work, talking to a colleague, when the first symptom appeared.

"I felt some really intense numbness in my left foot," Stefureac, 52, tells PEOPLE. "That numbness started creeping up my calf to my knee, and then up to my hip."

The sensation spread quickly, he says. "Throughout maybe fifteen, twenty seconds -- fast enough that it's kind of freaking me out. It moved up to my torso, my left arm became really rigid and I couldn't move it."

His coworker called 911 -- and over the next few months, Stefureac says doctors tried to figure out what was going on.

But it wasn't until he had an MRI in the spring of 2011 that they saw the telltale lesions on his brain.

"That's when he said, 'Yeah, you have MS,' " Stefureac tells PEOPLE, calling the diagnosis "a punch in the stomach."

"I didn't know anyone in my life who had MS. So, I didn't really know what it was. I knew it was something you don't want to have -- you know, one of the diseases with the initials."

MS, short for multiple sclerosis, is a disease where "the immune system attacks the protective sheath (myelin) that covers nerve fibers and causes communication problems between your brain and the rest of your body," according to the Mayo Clinic. "Eventually, the disease can cause permanent damage or deterioration of the nerve fibers."

Shortly after his diagnosis, Stefureac, an electrical engineer, said he decided to "get healthy" and at first, started running because "it helped me quit smoking."

But then, he said, "when I started jogging, I noticed that I limped and that was my most major MS symptom that I never knew that I even had -- because I had never jogged in the previous 10 years."

"The fear of the progression of this disease kicked in, and I decided that I would do everything I could to not let that happen. I thought that if I just ran more, that that would fire those neurons from my brain to my leg and foot and everything thousands more times. That's gotta make it better."

So Stefureac kept running, working his way up to half-marathons -- and then ran his first marathon, at Mount Charleston outside of his native Las Vegas, in 2018.

It was after his second marathon that he decided "let's just do something really cool."

"I had seen that there was a marathon in Antarctica. and I thought, 'Oh my God, that would be so awesome.' "

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He had already signed up for marathons in South America and Paris -- and in December of 2021, Stefureac made it to Antarctica. That's when he knew doing a marathon on every continent was possible.

A marathon at the base camp at Mount Everest checked Asia off the list last year. "That involves a two-week trek up to base camp, spend two nights at base camp, and then run back down," Stefureac tells PEOPLE.

After Mount Everest, Stefureac and his daughter, Madison, 20, ran together at Entabeni Game Preserve in South Africa.

Brisbane, Australia followed -- and Stefureac capped off his continental runs with a marathon at the North Pole this past August.

"It's all just a frozen Arctic Ocean and they set up a little track and we ran 26.2 miles around this around this track."

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He said that when you needed to pass a runner, you had to go into the food-deep snow outside the track.

And the entire time, "we had to wear life jackets." he says, due to the "weaker spots in the ice" and open water.

Now, not only has he run a marathon on every continent, but Stefureac has his sights set on the Boston Marathon.

"I believe my running really helps keep the MS at bay," he tells PEOPLE. "I have been symptom-free for quite a while."

"I have minor aches and pains," he says, but "some MS symptoms can sometimes just mimic getting older symptoms."

And although he jokes, "newsflash, diet and exercise is good for you," Stefureac adds, "it's really been an extreme case in mine. It's like, diet and exercise with a couple exclamation points."

But mostly, he hopes people realize that "people are succeeding out there, living not even just an ordinary life -- but an extraordinary life -- with MS."