iPhone Health "Walking + Running Distance" is way off

The "Walking + Running Distance" on my iPhone 6 is wildly off.

I've walked <2 miles today (0.5 mile commute, 0.8 miles for lunch, and whatever around the office), yet my iPhone is reporting 10.3 miles. It's been way off like this for a while.

Is there any way to troubleshoot what's happening here? Or recalibrate the feature?

Number of steps seems OK...6767 steps at 1.5ft/step would be about 2 miles, which is close to my estimate.

Additionally, I cannot "Show All Data" for any of the health metrics, the loading spinner just keeps spinning.

If you have any 3rd party apps for health tracking, like Withings, they sometimes add onto your records.

My experience was my Withings app completely doubling my steps and distances. If you do have these apps, delete them or just turn of their accessibility to your health.

The whole thing's weird... it says 7k steps but the top line shows 3k, Walk/run again, 10m, but the top line is only 5... & repeat for the flights.
The top line ought to be the next 'sensible' number above the highest single point recorded for that day/week/month.

I'd try at least a reboot if not a restore.
The issue with waiting forever to view the details may be related. I lost about a week of data when mine did that. The dotted lines are a giveaway that it's not reading its own data correctly.