I'm not sure why a healthy, able-bodied, w/o any physical or mental disabilities 8 year old would need a stroller.
When we first took our kids to WDW, our DD was 5, & older DS was 3 (almost 4). We took 2 umbrella strollers, & they rode in them most of the time.
The next time we went, they were 6 & almost 5, & we took the strollers again. However, by the 2nd day, we were leaving the strollers at our resort.
Strollers are a pain in the parks - it's so much easier to navigate the crowds w/o a stroller! Additionally, it's a pain to fold up a stroller for the buses.
Now that we have a younger child again, we're back to using an umbrella stroller - and it is nice to have a place to store our stuff - my bag, extra water bottles, diapers & wipes, etc. And, since he's younger, I like that he's contained & secure.
However, I won't miss the stroller either when younger DS is old enough to not need a stroller.
I definitely won't miss having to park the stroller somewhere & then coming back out of the attraction & having to backtrack to find our stroller.
Yes, WDW is big. But, honestly, if your child can walk through your local mall or zoo or amusement park w/o any problems, he should be fine at WDW.
As someone else mentioned, there's not a lot of straight walking - it's walking & then stopping to stand in a line & then riding & then walking some more & then stopping & then walking some more & then stopping for a bathroom break & then walking a little bit & then standing & then sitting down for a snack.
And getting in & out of a stroller & finding a place to park your stroller for all that is more exhausting than just walking.
Your 8 year old shouldn't be any more tired than you are.
Walking is a wonderful way to get a little exercise!