Hi @dieBrood,
first of all congrats for finishing a 100 hrs endurance event! Grea achievmentt! I understand that you want to show this to the world!
I was just looking around in the STRAVA Support articles and found this one: https://support.strava.com/hc/en-us/articles/221033867-Activity-Split-Tool
But I asume you recorded the complete 100 hrs in one file with your bike computer. I am not sure if it is because of the overall hours, but there is a max. file size of 25 MB for manual uploads of GPX/FIT files. Would it be an Idea to split the complete file into (min.) two acticvies?
If you google for “fit file splitter” you may find a tool for splitting fit-files. Make sure you have a backup of your original file.
Otherwise I would suggest to open a ticket at the STRAVA support and ask for some help: https://support.strava.com
Cross my fingers and hope they can help you!
Cheers!