The Apple Support link was not very helpful and just provided links to setting up my phone (which I know how to do). I did not try hard to find other options, because I solved it today myself.
I went to Health App -> Summary -> Show All Health Data and then scrolled down to the bottom and some app (not me) had changed my height to 2'0" and weight to 72 lbs even though correct height and weight were in Health all along. Next, I clicked on "Height" scrolled down again to "Show Data" and a new entry on 10/21 (when problems started) changed height to 2'0". All other entries in the list have a heart icon and are entries I made in Health app. This new one said it came for "iPhone" (with a phone icon) and did to say what app did it. I also checked my "Data Sources & Access" for both height and weight and both of list ONLY "Heath" for source allowed to update height and weight. Despite these settings some other source was allowed to make the change and that change took precedence of the Health app setting. I don't know how or why. But I deleted them, entered correct ones again to be sure, and now distance is OK.
Other things I learned are:
- Some discussions say Health distance is done by GPS while steps is by built-in accelerometer. Apparently, that is not correct. Health app distance is based on steps and stride estimated from height (or calibrated if you have Apple Watch, which I don't).
- I have been concerned later that my "Walking Asymmetry" was running 10%-30%, possible indicating slow recovery from recent back surgery. When I fixed my height, the asymmetry dropped to zero so value must depend on height as well.
If your Health app is unreliable, check "Show All Health Data" for unauthorized changes to your height and weight or any other settings.
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