Garmin malfunction: I walked 15+ miles 2 days ago. App recorded min/mile, duration HR correctly, but says 5.5 miles total

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ooops! done. thanks

The times look a bit odd. Where you out walking for almost 10 hours? Moving time is 3 h 15 minutes. So, you must have been standing still or moving really slowly a lot of time.

One thing that cause odd logging is the auto pause. Especially for slower activities like walking and hiking. Don't ever keep it on. It will just give you odd results. But with that said. It should also stop the timer.

there was also two hours of steep elevation where you can see the heart rate was elevated. Likely slow but walking still.

Check auto pause and make sure it is disabled. If it is on it can explain some of the issues.

The missing map is odd. Did you see that it connected to GPS? 

Some users experience GPS issues (like missing maps) and they are usually resolved by a restart of the watch.