@Cavasta wrote this question back in October. It was never answered and I was wondering the same thing.
"Is it possible to do a TR outdoor workout on a Garmin while following a route using the Garmin’s GPS/navigation capabilities? If so, what’s the sequence for getting going - load and start the route first followed by the workout or vice versa?
Scenario: I’ve created a route using Ride With GPS and transferred it to my Garmin. I would like to follow the route using the Garmin (using GPS, turn by turn navigation, etc.) while simultaneously doing a TR outdoor workout that has also been loaded onto my Garmin from TR. Can this be done?"