Corrupt .FIT file :(

solved

before the 'kind one' helps

Must be confusing me with someone else.

not to exceed 6 fit files at the most at any given time

Must add this to the "Forerunner 35 Tips & Tricks."

So, I think that it's done this for the same reason that a previous one of mine did - too much pausing! In a 'normal' run, I don't tend to pause at all but this was a long one and I kept getting quite lost!
Just to answer your other Qs - 
33 files in the Activity folder. I clear this out semi-regularly but clearly need to do it more!
The oldest of those files is 4th September and it was about an 8 mile run, so not a particularly long duration.

Thank you for the reply and I hope these answers help in some way! Thanks also for the tips!

This will work - B9RB0343-fixed.fit

Amended suggestions

• Manage the device's \GARMIN\ACTIVITY folder so as not to exceed 6 fit files at any given time by backing up then removing  the files on a regular basis - Removing Recorded Activities on a Garmin Fitness Device 
• Avoid excessive button presses such as pausing during your activity. If you need to pause frequently during your run, consider turning on auto pause
• Regularly check for firmware updates using Garmin Express
• If issues with corrupt files persist, contact Garmin Support

solved

Fantastic, thank you so much!

The list above is mostly a work in progress from contributions from JoeD, 4567890, Garmin Support documents, Alan Weatherall, and, Mathias Krallmann.