Hey there WHFT,
Thanks for reaching out to the Apple Support Communities. If having issues with the Health app not tracking your walking distance as expected, let's check a few settings that may help with improve this accuracy.
First, let's ensure that you have Location Services enabled using this link: Turn Location Services and GPS on or off on your iPhone, iPad, or iPod touch - Apple Support
After checking the setting using the link above, we'd also recommending checking that Motion Calibration & Distance is enabled in iPhone > Settings > Privacy > Location Services > System Services.
If you find that both of these settings are already enabled, then we'd recommend reaching out to Apple Support directly from this next link here: Get Support - Apple
Take care!