Dodgy distance tracker on 7S

Look in Garmin Connect. Does it say that you ran 50 km? Did the watch say you ran 50 km when you stopped the activity or did it log 50 laps?

Check the auto lap distance in the watch.

• Thanks for trying to help. 
  I gave the watch a Software update last night and a hard reset. 
  Then I ran for 40 mins this morning and covered 156.5km. 
  (which should have recorded as just 8km). 
  
  All details on Garmin Connect suggest I ran 156.5 km.
  Avg moving speed on Garmin Connect suggests 251km/h. 
  
  
• I checked your suggestion of the Auto Lap, but all seems to be in order. 
  
  
• don’t know what else to do!

Could you post a link to one of your activities? 

connect.garmin.com/.../11339747290

Thanks. Most values look ok, but he ascent is also dodgy. Could it be the altimeter isn't working and you got 3D speed enabled.

Disable 3D speed and see if that helps, but you still need to get the altimeter fixed. Contact Garmin Support. 

This is the link from this morning. 

You’re very kind Andy. 
I checked my settings and 3D speed was already disabled. 
(only 3D distance was enabled).