a few tips on the Garmins (here's the section for the 820:
frequent the Garmin forum often and learn from others as regards setting them up, and ask questions on the Garmin forum if you are unsure.:
https://forums.garmin.com/forumdisplay.php?591-Edge-820
don't download any new software until it's been tested .... Garmin often fix one problem, then another problem is encountered
I have the Edge 1000 ....
when I received it, the icon showing the turn by turn direction was off screen. This was a software bug, and a few days later, a new version of software was available to download. It fixed the issue, but now the 1000 has a habit of switching off a few times during a ride (not a big problem, as when you switch it back on, all the data for your ride is still there).... Garmin say that they will be releasing a new version of the software soon (I will wait before downloading)
There are a few settings that will give you more battery life if you plan to do rides longer than 6-8 hrs. Alternatively, you can purchase a portable power pack (there are small ones as big as a lipstick). If you purchase one of these, you have to make sure that the USB cable that you buy allows you to charge the Garmin while you are riding (some USB cables don't allow this)
I use ridewithgps.com (the free version) to plan my routes (in conjunction with Strava Heatmaps). RidewithGps is brilliant as you can drag the little yello google man onto the roads and see a pic of what the road looks like
You can also search google for cycling clubs in your area and check which routes they recommend