I suppose my first inkling came a few years ago, when I was still living in Highland, and a guy I knew along my jog route always seemed to be out puttering in his front yard, and as I chugged past would call out a variation of the same encouragement.

“Keep on smilin’!”

I remember thinking he was mocking me, because aerobic exercise for the sake of aerobic exercise has never been pleasing to me. Let alone fun. When you’re young, and you can get 45 minutes of serious cardio while playing basketball, well, that’s one thing. But when you’re too old to risk that rolled ankle or balky knee or back spasm, and you are reduced to some boring, repetitive, Sisyphean labor … no, it’s no fun.

If I’m doing it right, running/plodding is something I dread and find painful to go through. In fact, my entire mood can be altered from the moment I awake, once I figure out that 1) I have to exercise that day or 2) I do not have to exercise that day. The realization of the former often casts me into the depths of despondence … and of the latter, makes me feel as if I won some sort of prize.

So, I decided, he was mocking me. I so clearly was not having fun that he was messing with me. “Keep on smilin’.” Yeah, sure. And, see, he was one of those wiseacre kind of guys, anyway.

Now, I’m thinking, hey, maybe it’s about me.

There is a sort of brotherhood of joggers, isn’t there. I mean, we’re all over the map, in terms of ability. This guy is doing seven-minute miles, that oldster is barely moving faster than a walk. But we often acknowledge each other, don’t we? A nod. A hand raised. Maybe even a quiet “hi” as we plod past each other.

We’re engaged in similar activities — exercising or training — but we don’t bring the same mood to the event.

Some of those runners (not joggers) speak rapturously of the “runner’s high.” About endorphins flooding them, leading them to feel giddy, or whatever.

That doesn’t happen to me nor, I believe, to most. We might feel good at the end of the jog, but it’s mostly because we know that, right that moment, we are further away (in terms of time) from our next jog as we can possibly be. It’s over. We have done our duty, forced ourselves to perform and there is a grim satisfaction in that. But mostly it’s relief. “No running for the next 47 hours!”

So, anyway, when I am out there, I am not having fun. I am aching. I might be wheezing. I might be cold. I wish it were over.

So why have so many people met my glance with smiles on their faces? Some of them even in cars or on bikes?And why have they been doing this for years now? And why did it take me so long to notice?

Inside my head, I am convinced I must look just short of pathetic. Struggling. Uncomfortable. Maybe in serious pain. Because, often, I am all of those.

But my face must be arranging itself in such a manner that it appears I might be smiling.

In the mortician’s business, they call this a “rictus.” A fake smile that often seems to appear on the faces of dead people because of the way muscles settle or pull on the face.

Apparently, that’s me. While jogging. Mr. Rictus. I am miserable, and my face knows it — but it’s showing something else.

The other day, crossing a bridge back over from the Naples neighborhood, a woman in her mid-20s flashed a smile at me so broad that it almost made me laugh. I mean, 10 years ago I might have thought she had a weird thing for slogging middle-aged men … but now, I finally have figured it out.

When I jog/plod/slog … my face apparently settles into some sort of grimace that, to most people, appears to be a smile. Or at least a grin. And as they pass me, they often smile back at me. Cuz I’m having fun!

I am tempted to stand before a mirror and see if I can replicate my 20-minutes-into-the-plod face. But I don’t think I can do it. If I could, wouldn’t I have figured this out before now?

But I must be doing it. Too many people see my sad, shuffling figure … and smile. They can’t all be mocking me.

So, yes, that neighbor who I thought was taunting my suffering with his “Keep on smilin'” shouts? Well, apparently I was smiling. Or near enough.

Who knew? I do … now.