We recently completed this bike trip. It was a really wonderful trip. Great biking, fantastic scenery and the hotels were really terrific- def better than we had expected. The first 3 days were in the mountains and very picturesque. The last 3 days were along the river and we rode through a lot of cornfields and frontage roads.
We had no proper training beforehand and the days were reasonable - though we did walk our bikes up some of the steeper hills.
We booked an extra day in Krimml which allowed us to better explore the waterfall (which is amazing). There wouldn't have been enough time to walk all the way to the top if we had not. It was real bonus that the hotel gave us free passes! We also took an extra day in Salzburg to explore. That was a really nice way to break up the biking.
This is a really nice trip for active people and families with older kids. The bikes and other equipment that was provided was good. Though we found that people who booked directly with the operater - Eurobikes - were given GPSs. I don't remember that option - which would have helped because we missed a few turns along the way.