How do you tell a buddy he’s sadly mistaken?

One of mine told me he lost the desire to race bicycles, but plans to ride recreationally as a way, in his exact words, “to stay in shape.” But there is no human being on the face of this earth who can go from riding 150 miles or more a week with 50 or so of those at or near maximal effort to riding 100 miles a week moderately and maintain the same level of fitness.

No how. No way. Especially once you start, as my buddy has, your fifth decade.

What I believe my buddy actually means is actually true: that recreational riding will keep him in far better shape than the majority of those who also just turned 50 and don’t do as much exercise. What’s false is the belief that recreational rides will maintain his prior level of fitness.

Invariably, he’ll discover that this summer on one of those perfect-for-cycling-days when he gives into the urge to ride again with his buddies who race. The ride will start off fine, but about the time the third hour begins and the intensity increases, he’ll struggle, really struggle.

Struggle big time.

Now he may manage to stay with us by being especially bike savvy, soft pedaling whenever possible, and riding in the draft, but it will certainly be a physical and mental struggle. Moreover, a day or two later, he’ll still be so spent or sore or both that he’ll use that discomfort to justify why it really is better for him to ride less intensely since he simply wants “to stay in shape.”

But my buddy is sadly mistaken.

He’s mistaken because his initial goal is impossible. “Remain” is a suitable synonym for “stay” and what was established early in this article bears repeating: If you achieve a high level of fitness before you hit 50, you will not “stay” at that same high level by the middle of your fifth decade — even if you adopt the opposite approach and work out more intensely than before.

But with age comes wisdom and by the time most people reach 55, they accept that fact. What they shouldn’t accept, though, is not achieving that same good feeling from working out as they did 10, 15, or even 20 years ago.

To recreate that as you age, you need to drink from the Fitness Master’s Fountain of Youth. The beverage served there is a simple concoction and certainly good for you, but it’s definitely an acquired taste.

For a part — just a part — of every other or every third workout, exercise as hard as you can.

What someone who’s 50 can do almost as well as someone who’s 30 is work out at 80 to 85 percent. So what most who are 50-plus do is construct a regimen of workouts that never get any more intense than that.

Why that plan falls short is because by 50 or so even the most hardcore health-and-fitness fanatics start naturally losing some muscle mass. It’s called sarcopenia, and it’s as inevitable as skin losing elasticity or hair turning gray.

But whether you are 15 or 50, this fact remains the same: muscle doesn’t strengthen or grow unless it has been damaged through physical activity.

I was reminded of this last fall when I began a rather intense weightlifting program — but not with the idea of turning back the hands of time. I simply wanted to regain some of the power I once had on the bike.

And the program has certainly helped.

The reason why is simple. Even though I’m still losing muscle mass because of sarcopenia, I’ve regained what was unnecessarily lost by not occasionally going 100 percent in the weight room — and not going 100 percent nearly as often as I once had on the bike.

Yes, lifting all-out is tough, especially for someone like me who has far more slow-twitch muscle than fast-twitch. Yes, it can be depressing to use heavy weight in the weight room that’s no more than 70 percent of what heavy was way back when.

And yes, it’s really hard to ride up Hawk Mountain all-out and know it’s going to take me 45 more seconds to reach the top than 15 years ago.

But something else has occurred. Something far more important.

I really feel good again. More like a kid. Less like an old man.

And not just when I am working out, but as I work throughout the day. Getting up at a ridiculously early time seems easier, yet staying awake at night doesn’t seem any tougher.

In short, intelligently battling the natural physical decline that results from the passage of time has done more than slowed that unpreventable process. It has enhanced my quality of life.

Which is something I’d like you to also experience.

So, as long as you have your doctor’s okay, don’t be satisfied to do three sets of lat pulldowns for 15 reps the next time you work your back. Do one set with a weight that allows that and then add 10 or even 15 pounds, but shoot for three fewer reps.

And then add even more weight for the final set and do as many reps as you can.

My guess is that in the middle of a workday about three weeks from now, you’ll wonder why there’s more of a hop in your step, remember what this article suggests, and realize that you’ve really stumbled onto something.