Thanks to all of you who are chiming in on the heart rate discussion! I'm not overly obsessed with heart rate as a measurement, and I agree that it's quite variable due to factors like fatigue and heat, but it's interesting to observe how things change over time.
Here's some more self-indulgent nerdiness on the topic ...
When I was a collegiate rower (1985-89) our coach had us wear heart rate monitors for hard sessions. I remember that I was maxing out in the mid-180s and wondered why I wasn't hitting the 190s like some others in my boat. But I figured if my rowing ergometer test numbers were good that was better than worrying about HR Max.
After college I rowed with a national team development program, getting in with a very fast lightweight 4 boat. We won some national championships, and eventually every other guy on the boat got a national team seat ... but not me. I was always a step behind on the test scores -- not by much, but enough that I figured I'd be better off not banging my head against a wall trying to keep up. I pulled 6:28 for 2000 meters on a Concept 2 if there are any rowers in the crowd. Interestingly, the coaches never had us wear heart rate monitors -- the erg score was really the only measurement that mattered to them.
Later in life I started running regularly, and doing some cycling and triathlons. In my early to mid 40s I was doing weekly cycling sessions with coaches and watt-based power metering. The workouts were fun because they were done in a group setting, and really challenging. I was surprised to see that my max heart rate was now below 180, but the effort was just as intense, and other objective scores were pretty good. My functional threshold watts for a 20-minute test was 275 (at 160 pounds, and at an altitude of 5500 feet) whereas a top guy (of my size and in my age group) might be 300 watts or so.
Now that I've cleared the 50 years mark, I'm mostly running and doing some general fitness stuff. My brother got me a month of passes to Orange Theory, which is an indoor group workout that utilizes heart rate -- the goal is to accumulate points in "orange zone" which is defined as 83 to 91 percent of max heart rate. However, since the parameters for their heart rate zones are based on standard charts and not customized for each athlete, I have a hard time getting to the "orange" rating. It's a little comical because I'll be wailing on the rowing machine or running hard on the treadmill during the workout and I'm still the the "green" zone, while the person next to me is trotting along at a slower pace but has their score pinned in the "orange" range. But it's no biggie, because I can definitely tell by feel that the effort I'm putting out is challenging enough for me.