Take Me to the River

Skip the hotel gym the next time you're in Continental Europe: head for a jog beside the Danube instead. This khaki-colored river (Johann Strauss must have been either color-blind or extending his poetic license when he put "Blue" into the title of his famous waltz) is lined with hiking and jogging trails that make it one of the region's best-kept recreational secrets.

The 2,850 km-long Danube kicks off in Germany's Black Forest and flows eastward through 10 countries to the Black Sea . Once part of the Roman Empire's border against barbarian hordes to the north — and...