I finally broke down and bought myself a cadence sensor for my bike: a Wahoo cadence sensor, in fact. All I need is the cadence sensor and this little doohickey works nicely; it's small, attaches to the arm of the pedal, and syncs with the Wahoo app which, in turn, syncs with Strava.
The results were really eye opening, and I'm glad I bought it. This will really help me up my riding game. I had always assumed that I was riding with a cadence of about 70-80 rpm; it turns out that my average is much lower than that, which means, essentially I'm generating excess muscle strain for the same amount of wattage that could be generated with higher rpm's. In short: gear down a little more, pedal a little faster, expend the same energy, but in a healthier method which will likely also improve my speed.
Now, this particular route was not one I normally take, but I rode the same way that I usually ride. When I get back out on my standard lunchtime route I'm going to make an effort to keep the gear a little lower than normal and see how that works out.
Unfortunately the cadence sensor and my watch - which provides heart rate data - both sync to Strava but do so as two different activities, so they don't sync up together. I'm not too terribly concerned, particularly since the heart rate data from the watch probably has a margin of error of 5-15%, so I trust that mostly for baselines and average trends as opposed to firm numbers. The cadence sensor, however, does not lie.