Possibly need 3rd party tools. If you can zip the fit file and attach it or share it, we can take a look.
Zip the file, then in a reply post Insert>>Image/video/file>>click Upload>>browse for file>>OK.
Or upload the fit file to a reputable file sharing service, such as OneDrive, Google Drive, Dropbox, make the file public, then paste a link to the file in your post.
It's not that large. But the file is corrupted. Being from a Forerunner 35, this is not uncommon. This device often has issues with activities in the 10 mile and above range.
Some suggestions to help minimize the occurrence of corrupt fit files on a Forerunner 35
Avoid pressing stop or pause during an activity.
Sync your watch, backup the \GARMIN\ACTIVITY folder, then do a full master reset of your Forerunner 35 using these directions - Restoring All Default Settings.
Perform a weekly cleanup of the \GARMIN\ACTIVITY folder using these directions - Removing Recorded Activities to Free Up Space. The F35 seems to have problems with longer activities, so this step is important before a longer event.
