Walking from Durnstein to Weissenkirchen?

No personal knowledge to your question but found Google "Maps" useful in testing questions such as yours. "Maps" feature time and distance routings for walk, bike, train/bus and car.

"Maps" answer to your question is:
1 hr 9 min/5.5 km waling north of the river or
Same time and distance south of the river

As always mapping apps come with the usual warnings.

It is a little over 5 km from Durnstein to Weissenkirchen. At a guess maybe take you a bit over an hour to do it depending on your walking speed. I believe you would be walking along the bicycle path which goes along the Danube. As I recall when we biked that part, it was very pleasant.

I have walked, biked and boated this path. A quick question, though...you say "on the way back" so do you mean that you will go back to Melk after Durnstein? Is there a reason for this?

To answer your question, it is walkable, but you want to give yourself options in case of bad weather or extra shopping/exploring time. The WL1 bus runs along the north side of the Danube from Melk to Krems and back. Easy. In the end you might find you enjoy spending time in the villages rather than walking between them.

If you are coming from Melk, why not get off the boat at Weissenkirchen and then walk to Durnstein? I also highly suggest that you walk on the opposite side of the river and take the small ferries across at both towns (Weissenkirchen and Durnstein). The walk between Ruhrsdorf and Rossatz on the opposite bank is a nice one full of fruit orchards and vineyards. The path is well marked as the Marillenmeile (Apricot Mile) and here is a link:

http://www.marillenmeile.at/index.php?id=9

Thank you for your advice. We are staying in Melk that night which is why we are going back. Thank you for the suggestion of the Apricot Trail which is just what I was looking for.