I think it depends on the kid. I was a single mom and not a pack horse so my son was very independent. He did not have to hold my hand all the time, as a matter of fact the only time he had to hold on was if we were in lines at QS or when I was carrying a tray. He had to hold some part of my body then, my shirt or my pants leg or something that I could feel him holding on to when we were walking. Standing in line he just had to be in front of me. When we were walking around he just had to stay close enough to me so I could grab him if necessary. If it was too crowded for words, say after a parade or FWs, I would make him hold my hand or I'd just pick him up (and yes he was bigger than me after about 3 but it was only for a short walk and we managed). As far as wear and tear on little muscles, don't kids get out and run and play any more? I don't think my son knew how to sit on his bottom until he was a teenager, until that point even when he was eating he was perching on the side of the chair. He never once had a meltdown at WDW until he was a grown up adult man and he and his wife weren't getting along. The key for us was me paying attention to him and his moods. If I knew he was getting tired, we stopped and rested, more often than not I got more tired than he did. We went slow, no reason to run, nothing is going anywhere so it will be there when we get there. He would fall asleep in my lap on the monorail (when was real young there were only two parks, MK and Epcot so no buses. But then he fell asleep any time travelling in any vehicle and still does scared the crap out of me when he started driving but evidently he can drive and not fall asleep).
I think it also depends on the time of year. We have always gone June/July. The stroller provides shade and a place to sit and hydrate (we are more likely to have a drink immediately accessible if we have a stroller because they are easier to tote)
Maybe it wouldn't matter as much other times of year.
My children are on their feet a lot and go outside some, but 10 miles in 90+ degree heat is different (I've seen many adults laying on the ground with medical people around them while at Disney!)