Hello Oxymoronius,
Welcome to Apple Support Communities. We understand having inaccurate results when measuring distance you walked using the Health App. Accuracy is very important, and we'd be happy to look into this with you.
Which iOS version is currently installed on your iPhone? Settings > General > About.
To get started, please make sure your iPhone is up-to-date on software. When you navigate to: Settings > General > Software Update do you see any available updates? If so, please back up your iPhone, then update as explained in the following links:
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
Also, check out this helpful link:
Use the Health app on your iPhone or iPod touch
We hope this information is useful. Kind regards.