Why are visits to museums that tiring?

We visited a museum this afternoon and, as is always the case with me (and with my girlfriend too, by the way), after about forty five to sixty minutes I get tired.
Heavy, tired legs and a bit painfull shoulders are the most troublesome.
How can it be that I can be busy in my job (where I also walk quite some distance) for hours at end okay, we have a coffee break and not get (that) tired, but after not even an hour in a museum I feel like I ran half a marathon?
Sounds familiar to you?
Can it have to do with the fact that in museums we tend to stroll?
What gives?