Most of the outdoors activities in NZ are more strenuous than the similar ones in the US, fwiw. I guess the kiwis are a fit and active people, and the lifestyle reflects that. When giving directions people will tell you that it is "walking distance" or "a short walk" when it is 5 miles away, for example. Tourist oriented activities are there to stay in business, so they make it a good experience, but all these things require a certain degree of fitness. Bad knees would seem to be a problem (I wouldn't do it, from a max dynamic load standpoint). You could try emailing the operator and asking their opinion (they may or may not take this kind of risk, though in general NZ sports are also much higher risk than the average in the US).