I'll admit--I've been known to wear uncomfortable shoes just 'cause they're cute instead of comfortable not-as-cute shoes to work. And at the end of the day, especially if I wear heels, I'm in so much pain!
So, a couple of years ago, I bought a pair of Fast Flats; I keep them tucked away in my classroom in case I ever need to slip them on.
New teachers: make sure you have some back-up shoes in your classroom. I remember when I student-taught, my cooperating teacher kept a pair of slippers under her desk. She would slip into them during her planning period or whenever she had a few minutes at her desk. Trust me, you'll want some back-up shoes.