To manually upload the .FIT file to Garmin Connect:
- Open Garmin Connect in a web browser: http://connect.garmin.com
- Log in if you need to
- Go to the activities page https://connect.garmin.com/modern/activities
- In the upper right hand corner of the page you will see a link for Import, click on that
- A dialog box will appear, and then click on "Choose File"
- Find the .FIT file you want to upload, on Windows they are in the Document/Zwift/Activities
- Click on the file that you want to upload
- Then click on the "Upload activity from file"
Don't worry if you have you Garmin Connect account linked with Strava, the new workout will not overwrite the current activity that Zwift sends to Strava. I also did ask Zwift if they are planning to create a process to automatically upload to Garmin Connect, and they said currently they are not working on it.