My wife and I have always said we will save cruise trips for when we are too old for more active travel (We're 76 and 78 now), and that Viking cruise is on our radar. Please tell us about your experience when you get back.
We have done a couple of river cruises in Europe: Holland and Belgium and St. Petersburg to Moscow.
We've also done a couple of National Geographic/Linblad cruises: Galapagos and Antarctica.
The big ship cruises have no interest to us.