Are there any default settings on my Garmin I should change?

TrainingPeaks is not compatible with Smart Recording or auto-pause, as TrainingPeaks uses every second of raw data when calculating.  For the most accurate data, I recommend using every-second recording and turning off auto pause.

By default, most Garmin devices have Non-Zero Averaging turned on.  This setting throws out all the zeroes when figuring Average and Normalized power.  TrainingPeaks does not throw zeroes out because time spent coasting is resting, and rest allows you to hold a higher average power when you are pedaling.

I also recommend that you Autosync your Garmin Connect account and your TrainigPeaks account.  Checkout the TrainingPeaks FAQ on how to do this.