Imagine this: You get diagnosed with cancer. And not just cancer, but Stage 4 cancer.

Stage 4 means it’s advanced. It’s spread to other parts of your body from where it started. And frankly, there’s a chance you might not have long to live.

What do you do? How do you react? And how do you tell people about it?

I’m guessing this isn’t the first time these questions have crossed your mind. The American Cancer Society estimated more than 1.7 million new cancer cases last year, with more than 600,000 deaths. So it’s something we all face and fear — for ourselves, our family, our friends.

But let me tell you something: If I’m ever faced with a diagnosis like that? I’m going to try to live the rest of my days like John Andretti did.

Andretti passed away Thursday at age 56. He was diagnosed with Stage 4 colon cancer in April 2017, which means almost the last three years of his life were spent fighting this despicable disease.

The first 53 years? Well, that was when he was an accomplished race car driver, making an impact in a variety of series. Two NASCAR Cup Series wins, from tracks large (Daytona) to small (Martinsville). An IndyCar win. A Rolex 24 win. Even an NHRA national event semifinals appearance in a Top Fuel dragster.

Arguably, though, his final years will leave more of a legacy.

I say that with caution, because no one with cancer wants their life to be defined by a disease. But Andretti did so, so much since the day he was diagnosed that he truly made a positive difference.

If you’re an American race fan who hasn’t heard of the “Check It For Andretti” campaign (#checkit4andretti), I’d be surprised. Andretti was able to spread his message far and wide: If you’re age 50 or over, go get a colonoscopy.

He didn’t want what happened to him to happen to others. Had his cancer been caught earlier, there might have been a different outcome. So he decided to use his energy and emotion toward raising awareness.

Think about that: Someone with cancer already has depleted strength thanks to chemotherapy. They feel awful. There’s fear of the unknown, stress about how this all affects their family and friends.

And yet Andretti put so much into spreading his message — all in the name of helping others when he could have simply withdrawn and focused on himself.

We’ll probably never know the numbers, but you can be absolutely certain he saved lives. Even if only a small percentage of people heeded his message and took action in scheduling their colonoscopy, there’s no question some were surprised to find cancer as a result.

Maybe those people had been putting it off or weren’t even aware they needed one. So if they detected it early enough because of Andretti’s push? That, my friends, is making a difference.

But here’s something else about Andretti that is worth emulating for any of us facing a terrible situation like Andretti did: He stayed relentlessly positive. He was upbeat, optimistic, avoiding the “woe is me” attitude that would be very easy to carry. How many of us could do the same? I’m not sure I could, honestly.

In private moments, it had to be so difficult for Andretti and his family — wife Nancy and children Jarett, Olivia and Amelia — to keep their spirits up. But even then, John had strength.

As Jarett once told the Indianapolis Star: “What you see with him on TV is what you get at home. That’s the beauty about it. He’s the same person. It’s not somebody putting on an act or somebody that’s doing anything different than what he would be saying to us at home.”

In John Andretti’s tweets, interviews, or any conceivable message that would reach the public, it was always: This is happening, but I’m fighting, and I want you to watch out for your own health with me in mind.

On Sept. 7, for example, he tweeted, “I begin a new trial on Monday as the previous trial did not give us the results we had hoped for. In racing terms, we are changing the setup in an attempt to improve the car! I also have to make a quick pit stop with a surgical procedure on Friday. Please #CheckIt4Andretti.”

You can applaud Andretti’s on-track accomplishments and his unquestioned versatility as a racer. But when it comes to his lasting impact? It would be hard to argue anything was more important than what he did to help others — including the well-known Race 4 Riley benefiting the famous children’s hospital in Indianapolis.

Those of us still fortunate to walk this earth after so much tragic news of late should make sure to carry on Andretti’s message, even if he’s no longer here to do so.

(Top photo: Rusty Jarrett/Getty Images for NASCAR; inline photo: Doug Mathews / IndyCar Media)