Here are the steps to download a FIT file from the Karoo using a PC:
Enable Developer Options on your Karoo. Click here.
Under Developer Options, toggle "USB Debugging" on.
Connect your Karoo to your PC using the USB Cable.
On the Karoo, under developer options, tap on 'Select USB configuration'.
Select the MTP from the list.
(Tap on MTP from this list even if it is already selected)
You will now see the Karoo inside the File Explorer on your PC. Open it.
Now, open "Internal Storage".
You can now see all the folders inside the Karoo.
Open the folder named "FitFiles".
This folder will contain FIT files of all your Activities that are saved on your Karoo.
You can copy the desired activity to your PC.