and if you are on iOS, complain to Garmin because you can’t import into app. That is something Garmin needs to add to the iOS app (on iOS other apps like Cyclemeter and Wahoo allow you to import a fit file).
On RideWithGPS if you have a route pinned or copied to your routes, the easiest way to get on your headunit is to install the ConnectIQ app RideWithGPS onto your 830. Then just open the RideWithGPS app on your 830, and select the course.