The Austrian Danube - THE classical cycle path in Europe
The Austrian Danube is possibly the most famous cycle route in Europe. Most of its travelers cycle downstream, starting in or near Passau and ending their bike tour in Vienna. Some go even further to Bratislava (Slovakia) as it is only 35 miles away down the river. Just to avoid any confusion, Passau is in Germany. From here to a point 17 miles down the river, the Danube represents the natural border between Germany and Austria.
Our recommendation: bike the Danube downstream. Otherwise you would not be able to enjoy it as you had to look out for oncoming cyclists every minute. Approximately 38,000 bike the route every year between Passau and Vienna. Assuming that the majority cycles between May and September, that's about 300 people per day. If you cycle the Danube downstream, you will face a fraction of those only. You will be able to relax way more compared with an upstream itinerary.
You may also think about extending your bike tour at the beginning. If you start in Regensburg (Germany), you will enjoy another 100 miles to get rolling. Regensburg got a wonderful mediaeval town and the cycle path to Passau is flat and less traveled.
Find more detailed information about selected stages of the cycle path in Austria here: selected stages in Austria