(CNN) -- My heart was pounding so hard, it felt as if it were trying to break free from my body.
I couldn't breathe.
I felt dizzy and feverish, and my eyes stung from all of the sweat dripping into them.
And as I was desperately trying to figure out what was happening to me, I suddenly had this debilitating thought: My God, my son is trying to kill me.
Why else would he be running so fast? And so far?
When my 15-year-old asked if I would go jogging with him, I didn't think anything of it. We've worked out together many times before, and though it's been a while since we went running, I play basketball and tennis every week, so I'm in great shape ... for a guy my age.
But something unexpected happened somewhere between me laying him down in his bassinet and me being on the cusp of lying down on the sidewalk I was running on: We got older.
I had seen signs of this "older" before: a gray hair for me, a deeper voice for him. Our socks have become impossible to differentiate, and he has stolen a handful of my ties. But I still get carded at bars, and he still needs me to drive him to school, so I thought I was many moons away from accepting the notion that my little boy was not so little anymore.
Funny -- my dream for him is to be better than me, and now, as I hunch over, sucking wind with my hands on my knees and sweat racing down my face, it seems my dream is coming true.
I don't have a daughter, so I can't really speak much about that, but I can tell you it's a bittersweet experience for a man to slowly watch his physical superiority over his son fade to black. True, a large part of it is ego, and I'd be lying if I didn't say so. But there's also this profound sense of loss that comes over you as you realize that whatever it is you had with your boy is gone.
The love is still there, obviously, but the relationship has changed because the way you view each other has changed. I'm sure I'll still be the one he calls for help, but those kinds of calls won't be coming as often anymore.
That's not a good thing or a bad thing, it's just a thing that happens when the father's 40-year-old knees begin to crack and the teenager has started to grow facial hair. I thought we were simply going for a run that day, but it turns out it was a relay race instead, as I handed the baton over to my son and told him to fly.
"Dad, come on, why are you running so slow?" my son asked.
"Go ahead, son. Run as fast as you want. ... I can't keep up," I responded. Then I glanced over to my left and noticed a park bench. "I'm going to sit here and rest a minute. I'll get up and run back home with you when you make your way back."
"OK," he said, and then he took off, disappearing out of my sight. That day, I learned the deeper meaning of fatherhood: I may not always be able to go where he goes, but I'll always be there when he makes his way back.