Apple Watch Series 2 is reporting way too much exercise
Turns out that this was being caused by a timer app I had installed, called "Intervals".  I'm not using it as a fitness timer, but it defaults to recording every use of the timer as exercise.  According to the developers of the app, there are individual settings within each timer to turn this off, but apparently those aren't quite enough. There is also a "low power" setting within the timer, that needs to be enabled to completely prevent fitness tracking within the app.