This "pacing" thing is arcane stuff that isn't all that easy to describe to a beginner, but anyone who has hiked a lot and paid attention knows their own pace.

For me, what I concentrate on is my breathing. It regulates my pace, because it is tied directly to my conditioning and my conditioning changes from hike to hike and for longer hikes it even changes within the hike.

A good tip is to shorten stride on any uphill, so you can keep the same rhythm as usual and not overly labor your breathing. Deep breathing, with attention to pulling air deep into your lungs and somewhat accelerated exhaling, can propel you a bit faster and farther than if you just slog along and pant for breath.

Your ability to distribute oxygen from lungs to blood and then pump the blood to your extremities is the bottom line. Breathe control is where it all starts.