The Garmin 910XT is the premier GPS watch for triathlon training and racing.
You'll probably need a newer version of pyusb:
There is no need to install it systemwide - you can simply leave it checked out somewhere.
Get the Garmin-Forerunner-610-Extractor project from GitHub:
Turn your watch on and plug in the USB adaptor.
You can now sync your activities:
The first time you sync, you will have to authenticate on the watch itself. Downloading the activities will be much slower than you expect.
After the syncing has finished, don't forget to turn the watch off. If you are used to the Garmin Edge 800 (which turns itself off when you insert the USB cable) you will probably not have the habit of turning off the unit.
Rather confusingly, your .FIT files are available in the Xdg configuration folder. For example:
Strava supports these .fit files through the From File upload option.