laura, I'll just give you a quick resume of what I did a few weeks ago...I took the train to Krems and checked into the Bicycle Youth Hostel there...then the following morning I rented a bike and rode down the Danube to Melk in the morning/early afternoon...although my intention had been to ride back on the other bank via Spitz and Durnstein the rain was so bad I had to take the bicycle on the ship to Spitz and then take the train back to Krems via Durnstein and Stein...
Just reading back your post I think you're seriously underestimating the amount of cycling you can do per day...I cycled the 34km from Krems to Melk in approx 3 hours and also stopped off for coffee at Aggstein on the way...this is only the second time I've been on a bicycle in approx 20 years so it's not a matter of fitness or speeding along I can assure you...also the path is dead level with very few inclines...
So I would recommend you to do something similar...Krems itself is a lovely town and the suburb of Stein is also very charming and worth some exploration...so if you arrive there on Day 1 you can just immerse yourself there, gather your info for the following day etc...the hostel is a good if basic one with a separate shed for bicycles...
If you're cycling from Krems to Melk there is a map at the tourist office which details all the local ferry points etc...the most important thing you have to remember is that between Melk and Spitz the ship is the only way to get your cycle back as the buses do not take cycles...the last ship leaves Melk at 16.15 so you need to be aware if you miss this then you are committed to cycling back to Spitz (approx 20kms)...from there you can take your cycle on any train for a small surcharge (€2 I think)...Durnstein is really medieval and worth a stop which you can do by train and then resume to Krems (trains til 8.30pm or so)...
At Melk I would suggest you try and leave yourself at least 2 hours to visit the abbey properly and get a look around the town...3 hours would be ideal for both...although you can walk around much of the abbey and its grounds for free it's worth paying the extra for the tour and acccess to the library etc...
In Vienna there is Pedal Power near the Prater...you may be able to rent a bike there and drop it off somewhere in the Wachau but it may be a more expensive method then renting locally when you get there...
Don't hesitate to contact me by PM for more help as I have all the info and prices from my recent trip fresh in mind...
It's a great way to spend a day or two if the weather is right so good luck with all...
