I told my dad I couldn’t do it. Just didn’t have it in me.

What’s the “it”?

Performing the act euphemistically referred to as euthanasia. Ending a life. His life. Killing him.

The discussion took place a number of years ago after my father had visited an old acquaintance in a nursing home. The poor guy could no longer do anything by himself.

Had to be fed. Had to be washed. Spent his days in a diaper. Yet he still had a sharp mind. Sharp enough to know that the way he was living was no type of life.

“If I ever get like,” my dad said, “you know what to do. Sneak in one day and put me out of my misery.”

I felt no need to explain my morality on the matter. I simply looked him in the eye and replied, “No how. No way.”

OK, so maybe the intro is even more depressing than the title of this article. But the end of your life doesn’t have to be. You don’t have to end up like that guy in the nursing home.

You can, in a manner of speaking, die properly.

All you need to do is what the medicos call “squaring the geriatric curve” — something that’s far more likely to occur if you exercise regularly and vigorously and eat properly.

Picture a graph where the word “Health” is placed beside the vertical line and the higher the mark the better yours is. A number line beginning with 20 and representing your age serves as the horizontal line at the bottom.

If you chart the life of a genetically cursed couch potato who prefers the ones he eats with his cheeseburgers to be French fried and not baked, his health line will begin to descend even while he’s in his 30s. Eventually, the drop will curve enough so that it looks like a quarter of a circle.

And when it touches the bottom line, meaning the couch potato has died, it does so somewhere between his late 50s and early 60s.

If you chart the life of someone who makes healthy living a top priority, however, his life line in no way looks like a quarter of a circle. It looks more like one corner of a rectangle.

The line starts nearly straight — or even ascends a bit as he goes from his 20s into his 30s and even into his 40s. The line may decline slightly as it moves from the 50s into the 60s and 70s, but it’s still higher than the couch potato’s line in his 40s and 50s. The line declines just a bit more in the 80s until suddenly dropping and running parallel to the health line somewhere in the 90s.

What this graphing represents is the life of someone who was self-sufficient, fully ambulatory, and remarkably healthy right up until the time he passed.

And the passing? He simply didn’t wake up one morning. That’s how to die “properly.”

No protracted hospital stays. No time spent in an old folks’ home.

Instead, you stay unusually active compared to most people your age. In fact, you just might keep competing in track and field meets, traveling to every single book fair in your county, and writing succinct and compelling letters to the editor as someone I know does — who’s well into his 80s.

Your best odds of leading such a life, doing what the medicos call “squaring the geriatric curve,” requires more than good genetics. It requires you make your health a priority. It requires you to take pains to take care of yourself.

It means you do more than merely mouth the Fitness Master’s mantra: exercise regularly and vigorously and eat properly. It means you do both. Day in. Day out.

While I’m well aware that this thing we call daily life often does its best to shut down the roads that lead to healthy intentions, you need to recognize roadblocks in advance and go around them — while still taking side trips to take care of your day-to-day business.

For instance, more than one parent who makes child rearing the absolute number-one priority has approached me and said, “I’ll worry about working out, getting enough sleep, and eating well once the kids leave the house.”

To make such a sacrifice, obviously, these parents love their kids. They are consciously denying themselves a higher quality of life so that their kids can have that, possibly by participating in travel-team sports or classes in graphic arts, music, or computer science — all of which require parents to work a second non-paying job that too often winds up being more than part-time: chauffeur.

But these parents need to consider the type of life they want for their children when their children reach middle age.

Do they want them becoming the new chauffeurs, driving mom and dad to doctor appointment after doctor appointment — or even worse, driving three times a week to see mom and dad in the old folks’ home?

Sometimes you have to be a bit selfish to be the best type of selfless.

The type that’s long-term.