I agree. Some, like WWII can involve marching/walking/running all over God's creation (especially if the commanders are idiots and keep "running tot he sounds of the guns"). Some, like WWI can have periods of inactivity followed by sudden spurts of heavy activity.
Roman, well, you're wearing armor... with the new needlefelt combat, it can be very intense. Often though, it's more displays
Those are the ones I do. However, I have watched other time periods such as Rev War, where the events I watched were not particularly strenuous looking. March out, shoot some, possibly die (being careful to gently lie down when dieing...).
I guess it just depends on the event.