If you use the Garmin Connect app to upload your data to the Garmin Connect website then you can have these automatically synced across to your Map My Tracks accounts.

Connect your accounts

To set up the automatic sync between Garmin Connect and Map My Tracks go to Upload > Connect to devices and complete the connection process.

Once the accounts are connected Garmin will send any data from the last 30 days over to your Map My Tracks accounts. This can take a short while to complete in full. 

After that, each new activity uploaded to Garmin Connect will automatically be synced to Map My Tracks. the sync process generally takes a few minutes each time.