Dave - sadly we've encountered just this same thing on most of our Viking tours that involved walking, even demanding tours. Sadly, perhaps it is hard for people to realize they are not as spry as they used to be - the mind thinks they still have it, but the body can no longer keep up. This causes frustration on those who can't keep up and on those who do wish to have a demanding tour as described.
As Jim so accurately states: "Accurate Self Evaluation is one of the best skills for a traveler". We all need to revisit that, as painful as the answers may be. 🙂