Lwien, Kudos for running consistently. Exertion is a "percieved" sensation and not really
quantifiable. Your heart rate is being measured and after 2 miles the HR increases but
the pace seems easier. Could be your endorphins flooding your bloodsteam making you
feel good. It could also be your muscles getting nice and warm and falling into a
comfortable pace.
The exertion that I mentioned isn't based on perceived muscle fatigue or a sense of well being from endorphines, but rather it's strictly based upon breathing. Early in my run, I am huffin' and puffin' a LOT. But then, after the 2 mile mark, by breathing eases up, but my heart rate rises to where I want it. Happens every damn time. I've been chalking it up to my body trying to get enough oxygen in my blood to feed my muscles and my lungs are really working hard to do that until things warm up. Don't know, but I've always been this way.
I remember one time I had a stress test on a treadmill and near the end, the Doctor said, "That's weird", which kinda got my attention. Being that my HR was maxed out, I breathlessy said, "Huh? What's weird?", to which he replied that 10 seconds after my heart rate leveled out at max, it actually started to slow down while keeping the same pace and level on the treadmill. Wonder if that has anything to do with what I mentioned above.
I tried using HRM's and for me personally IMHO it was a waste of money. I'm quite mindful of my body and exertion and know what different levels are supposed to feel like. I can just tell by my feeling of exertion, breath, etc. and I always am just mindful and listen to my body.
For me, at 68 years old, running while carrying 220 lbs with me, I figure it's better to be safe than sorry, therefore, an HRM makes a lot of sense to me right now. Also, on hot days, like yesterday when I was running in 90 degree heat, there's no way in hell that I could guess my heart rate being that at those temps, my HR increases by about 5 bpm above normal, and even though my exertion level may feel low, my HR could definitely be too high. Harder to tie in exertion level and HR when running in those higher temps.
For me, an HRM is just a tool, to help me work hard when I need to and slow down when I need to. Kinda like having a training coach along for the ride. Some may call it a crutch, but it is one that I welcome along the way.
Could I run without one? Sure, but I'd just feel a little less secure not having one, and therefore, may, especially on cooler days, undertrain or on warmer days, overtrain. But.........there are a LOT of people out there who never use one and do just fine.