Hahaha I totally understand! Just the other day I wore heels... they were a size too small (I had borrowed them) so they hurt terribly! It was only for the evening though - but I would assume that's kind of how it would feel even in comfy heels if you wore them for a long time.
Forgive me if the answer to this is obvious, but why would you have to wear it for over 10 hours a day?
Makeup and costume application starts about three hours before the convention. The convention starts at 10am, my boyfriend wanted to be there AT 10am. Then we all stayed in costume until they kicked us off the convention floor at around 7pm. Then maybe you want to go party afterward? Or do like I did and yank the mask off my head as soon as I got in the car at around 8 at night. A costume like this with the full body suit, skin paint and latex mask is a freaking commitment!