I can attest to marathon bike rides (5+ hours, 100+ miles) and marathon XC trips (4+ hours) -- I rarely have to take a break for output, while I am certain to take care of input all the while.
I can attest also that hiking seems to having me taking care of output more frequently then the other two. Not sure why. It's much less effort to find a convenient place for sure. I can hypothesize that it is because of the nature of hiking where I stop to take in the view, photos, wait up, or otherwise rest on occasion, where perspiration slows down, allowing the bladder to fill up. I tend not to stop while cycling or XC skiing. I've never hiked with my heart rate monitor, but I suspect the average heart rate is lower (I don't tend to "hammer" my hikes like I do cycling and skiing), so this may be another cause -- lower intensity, less cooling required.
I can say in all cases that I sweat out the majority of my H2O while exercising, and so I am not at all surprised I don't have to pee it out.
I suspect in my own case the increased output post-activity is as a result of insuring I am re-hydrated.
Tim