I'm a long time Forerunner 305 user (it's ancient but does the work). I've been trying to migrate to GarminConnect and apart for occasional hiccups with synchronization it works rather ok... but I've face a problem recently - how to use a plan/calendar - with GTC it works just fine - I put workout on a day and then send it to device.
Why I wanted to use GC is that it have quite nice workout routines, so I selected one, added to calendar, synced... and nothing changed on the device.
Does GarminConnect supports only newer devices?