Alexys01--I think it is at least in part a product of age and what mountains folks call "cattiness" (a sort of catch-all term to describe fitness and level of vigor; i. e., "I ain't as catty as I used to be"). When I was a boy and right on into my 40s I couldn't get enough hours astream. In fac,t the first time I went to Alaska, where there was way too much sun, I about killed myself fishing 20-21 hours a day. Now five or six hours is usually enough for me, especially if I have an appreciable walk afterwards. Accordingly, I try to plan my fishing to be on the stream during optimal times. Another factor is the difficulty of the wading and terrain. In a stream with an easy gradient I'll not tire out as quickly. For me it is easy to tell when I've had enough. My reflexes wane, I'm less certain on my feet, and there is a dimunition of my level of enthusiasm.
Still, as my 100-year-old father says, "son, you've had a marvelously misspent life."