My dad thought he could wear his good, comfy Dr. Scholls sandals in the park all day, no problem. Well, he got through the day ok, but come the next morning, it hurt to even put his foot down, much less walk and by the end of that day, some nice blisters forming. I come in handy by having been prepared for this by bringing moleskin, some scissors to cut it to size and plenty of blister band-aids, both the big and the fingers and toes ones. He ended up grateful that I packed them and kept some in my purse.
My mom had kept switching between shoe insoles during the trip because her foot was hurting (we found out not long after we arrived home that she has arthritis in her foot - she didn't have time to see a foot dr. before the trip). She was also getting blisters because of it.
I kept one pair of tennis shoes on, only 1 set of insoles and I had no foot or blister problems. By the end of the 3rd day, my knees were starting to bother me, but that's normal. But I did bring another pair of sneakers as back-up, as i do on any vacation.