I recently did Black Water Rafting at Waitomo (3 hour trip). I felt the most tiring aspect was the paddling and by the end of the trip my arms were worn out. However, I'm not as young or as in shape as most of the other participants. My eyes must not be as good either, as I seemed to struggle with the darkness more than others

As far as walking, the short walk down into the cave and the short walk out of the cave are the only times I can recall where knees would be stressed. Inside the cave, the walking portions are kind of half walking, half floating so they are not so bad. Of course, there are certainly opportunities to twist an ankle if you are not careful.

The one thing that really bothered me about the tour was how disorganized it was in terms of equipment and the difficulty in find the right size. My socks were way too big, the boots big, the wetsuit small, the tube too big etc.

Overall, I felt is was a neutral experience. Not a waste of time/money but not a highlight of the trip. Zorbing on the other hand was ^ ^ ^

P.S. Do not stay at Waitomo. The lodging & food quality there is low.