Hi Heather!
Although my daughter was certainly capable to walk around the parks when she was 5 years old, we went ahead and rented a stroller while at Magic Kingdom and was certainly glad we did!
This allowed us (and daughter) to pace ourselves as we walked through Magic Kingdom, which is massive! At 5, she also still was not quite out of that "take-a-nap-in-the-afternoon" stage, so it allowed her to still take a nap while Mom and Dad walked around, shopped, ate, etc.
Other than that, there are bench areas placed all around the Park, if you want to take a breather. But the stroller worked great for us. Now, if they can only find adult strollers for Mom and Dad :-)
Have oodles of fun on your trip!