From Prag, take the Elbe Bike Path into Germany all the way until Wittenberg. At Wittenberg, switch to the Europa Radweg R1.
This is an awesome map book with the best maps and all the accomodation info for every spot, including where to eat and find groceries as well:
http://www.esterbauer.com/buecher/ht...1_frameset.htm
The attachment below is the profile for the route from Prag to Wittenberg. 377 km long, and not hilly at all. Just one tiny section where the path leaves the river bank.