You can download a map of ATH as PDF here:
http://www.athensflights.gr/index_maps.htm
Just click on the images.
If I understand you correctly, it's gonna be fairly simple.
Leave the Non-Schengen part of the airport, go to check-in to drop off your bag, enter the Schengen side of the airport and wait for your flight.
And last but not least, have a great time in Greece!
Distances are generally short (relatively speaking).
Since you've got a lot of time there shouldn't be any need to rush.