One Sunday I woke naturally at 3:45 a.m. and immediately had this thought: “I feel good. Really good.”

I perceived no pain near the top of my right hip where the two titanium rods inserted to stabilize a severely fractured femur end, not even the dull ache that is typical the morning after a hard bicycle ride. I swallowed once, twice, even a third time, yet could not create the scratchiness in my throat I felt occasionally after said ride, a scratchiness that is too often the harbinger of a nasty head cold.

The walk to the bathroom gave no indication that I had ridden for nearly four hours the day before. I detected no soreness in my quadriceps, hamstrings, or glutes. Not even a crick in my neck.

How good did I feel? So good that if I wasn’t so fanatical about keeping my sleep patterns consistent I would not have gone back to bed.

When I did rise for good at 5:00 a.m., I actually said aloud what I was thinking: “Man, I really feel good.”

My second significant thought of the morning concerned the novel my seventh grade classes were currently reading, The Giver, where precision of language is a priority in the Community.

The main character, for instance, recalls a time when just before lunch as a four-year-old he claimed to be starving. His teacher overhears him and takes him to task, stressing the difference between hunger and starvation, emphasizing that no one in the Community is — or ever was — truly starving.

When thoughts number one and two slam danced in the mosh pit of my mind, the collisions created a third, the title of this column: Exactly what is “feeling good”?

For you to have the health and fitness that allows you to thrive rather than simply survive — and particularly for peace of mind — you have to ask and answer that question — and repeat the process regularly. The reason for the repetition is simple.

Things and your feelings towards them will change with the passage to time.

One of those changes, for example, kept me from feeling “really good” for a few months about five years ago. For years I was hesitant to share the details behind it because it embarrassed me, but admitting it here might help you now.

Quite simply, I stopped getting the same sensations from working out, especially racing the bicycle, and I thought it meant I had lost my love of both.

Prior to the change, I could simply think about an upcoming race, visualize flying up its toughest climb or breaking away as the hill crests and instantly feel what many call pre-race jitters or butterflies — even though the race might be weeks away. And at a race as the race official gave the last-minute instructions, my heart rate would rocket, my legs would twitch, and it would take all of my power not to scream out, “For God’s sake, just start the blanking race.”

And then I lined up to race one day and — nothing. None of that happened.

I thought I had lost my love of racing. But I hadn’t really.

My body was changing. I was aging. That overload of hormones that used to get me so hyped up just wasn’t happening.

Worse, the aging process was making me feel as if I had lost my desire to push myself in training. In my 30s, for instance, all I needed was a 10-minute warmup on the bike to get my heart rate where it needed to be to climb a hill all-out or ride at race pace on flat roads.

Now sometimes I needed 60 minutes to replace a feeling of lethargy with one of “Let’s go.” At first, I attributed this to simply not wanting to push myself as hard as I once had. But that wasn’t the case, really.

I was making the mistake of equating effort with heart rate. Heart rate, however, decreases with age.

It took me too long to realize that no matter how many steep climbs I pushed until my legs went wobbly and my lungs hurt, I was never going to see 182 on my heart rate monitor again.

Until I recognized that age alters the body’s response to physical effort, I felt like one of those cyclists I often encounter — dressed in a $500 kit, sitting on a five-figure bike who create some lame excuse for why they suddenly disappeared when the ride really got hard.

There were even times when I felt like a poseur for putting my name to a column called “The Fitness Master.”

I hope this confession makes it clear why I needed to redefine what I considered “feeling good.” And why you will sometimes have to do the same too.

“Feeling good” at 58 will never feel the same as “feeling good” at 42. Thinking otherwise will simply make you miserable.

Now, for instance, I don’t see my struggles during a warmup to elevate my heart rate as a lack of desire, but take pride in trying again and again and taking the extra time for a proper warmup — when someone else would just give up and do an easy recovery ride. If “feeling good” takes 50 minutes more than before, that’s not going to keep me from achieving that feeling.

Or thoroughly enjoying my time on the bike.