Could you please make it possible to mass delete workouts?
You can mass delete workouts if you connect the watch over the USB cable to a PC. The workouts are in the folder //GARMIN/Workouts. With a Mac it is possible too, but if your watch uses the MTP mode for the USB connection, you may need to install an application for accessing MTP USB drives.
Otherwise, if you want it directly on the watch (or done through the app), I recommend suggesting it to Garmin at Share Ideas | Garmin.
