The Danube River flows from its source in Donaueschingen in Germany’s Black Forest all the way to the Black Sea on the Romanian coast. 

This particular route starts at the source and follows the river along a dedicated cycle path, all the way to the heart of Vienna. A total distance of 565.91 miles (or 910.74km). 

A nice 100 km+ route, starting at the border with Belgium, in the town of Schmëtt, all the way down across the Capital Luxembourg City, to finish on the border with France in the town of Rumelange. One version is mostly asphalt roads the other version is using forest trails to a large degree.