Hi,
I just got a 920xt. :D Before that I used my dear old friend named 305 with Sporttracks 2.
Is there any way I can import all of my old activities into Garmin Connect?
This is actually easier than you'd think. You will probably lose all of your notes, but the process will be very simple. The answer? Use tapiriik.
You can set it up to do a sync from SportTracks to Garmin Connect to get all of your old data copied over, and then you can reconfigure it to copy any new data from Garmin Connect to SportTracks.