Not everything that can be counted counts, and not everything that counts can be counted. Albert Einstein, (attributed) US (German-born) physicist (1879 - 1955)
The fine folks who make exercise feedback devices - heart rate monitors, running watches, distance and activity trackers - do a great job at collecting and displaying raw data. The devices are smaller, more independent and able to learn what is quote-unquote normal for the individual wearer.
Honestly, would anyone now put up with the first consumer GPS receivers? The first versions I saw my old training group teammates wear years ago look absolutely gargantuan when set next to the latest fitness trackers. And those big things couldn't track your heart rate.
I think I raved about a year ago about the ANT+ wireless communication protocol and how I could communicate from my heart rate chest strap AND the stationary bicycle I was working on at my spin class. Naturally, that rave turned into a rant once I had battery issues and the inevitable (sweat-related) problems. Buy a less-expensive (read: cheap) Chinese-made strap, just to learn the age-old adage:
You get what you pay for.
Fast forward to the past month. Tracking exercise efforts based on average pace works okay if you're not in the need for really-granular information. And in the case of this old coach/guru, the less-granularity, the better. Until you decide, after a few months of running...without walking like an zombie the next day...that it might be time to dabble at racing. So there was a need - again - to know just how hard I was going, and more importantly, when to stop.
Lately the fitness devices have; added stuff which you couldn't track without going to a laboratory and paying a couple of hundred bucks. I didn't need that information because I had a test about eight years ago; that sort of baseline data doesn't change much over time. At least not in a drastic manner. Even better was the fact I found a device which didn't need a chest strap. Great! That means in the event I want to scare off little old ladies and small children (by running shirtless like I used to, so long ago) I could get my "half-unclothed serious distance runner in training" on I could do it without looking like an absolute geek.
Even with technology improving by leaps and bounds - providing everything to the point of estimating when you could repeat the workout which just finished kicking you in the butt - it's still a number.
Repeat after me: Distance, pace, heart rate, vVO2, EPOC, training Intensity, and so on, are data. Numbers.
The most important thing is to know what those numbers mean to your body.
How does my body feel at a particular pace? How do my legs feel after a particular distance? How long does it take for me to recover from a workout at a particular effort level? If my heart rate monitor reads 'x' and my legs feel 'y' and my lungs feel 'z,' then which of the following do I trust and act upon? The heart is a demand pump. The oxygenated blood is sent to the places the body informs the brain it is most needed. That means there's a lag time between an effort is performed by the muscles and when the blood arrives to replenish the muscles...kind of the reverse of the way our automobile's carburetor works. Depending on hydration (or dehydration), weather conditions, fitness, etc., a particular pace effort can vary in heart rate.
Feel "good" but the heart rate is higher than you'd like: If the workout is not planned as a hard effort, then I recommend backing off. A six-minute-per-mile pace is not always going to equate to a 145 beat-per-minute rate, just to give an example.
Feel "bad" but the heart rate is lower than typical: So, last week I had a ten mile long run on the plan; based on where I turned around I ended up getting eleven, with the last four miles taking a great deal out of me. The next morning's run was supposed to be an easy 3.5-to-4 miles, depending on how I felt. I got two very dead-legged, chest-heavy, heart rate-light miles in and decided to call it a morning. That evening's run was a slog, as well as most all of the easy efforts during the week.
Thought I had it figured out completely by yesterday morning's eight miles, but my body told me I was still fatigued by way of a high heart rate during a long easy walk. A trend of fatigue is pretty much a warning sign of overreaching. Push through that too quickly and the next stop is most likely overtraining.
When gauging how hard your workouts need to be, or when it might be necessary to take an extra day of rest, heart rate is only one number to measure over time. How's your sleep, your work stress, your hunger and thirst, and your diet? Another data point to consider is how you feel going into the workout and how you feel mentally coming out of it. Not everything can be measured to that little device.