Having a backup pair of shoes is probably not a bad idea. I did this for years, and what I discovered is that I went to the backup shoes exactly once, and that was when a sole completely detached from one of my main pair of shoes. The backups, however, were never flats but rather lowish mid heels that I know will never bite me back. My test used to be could I work a farmer's market in a give pair of shoes, a venue in which I was required to stand pretty much continuously for five hours, and that's pretty much the maximum that I'm in heels at one shot these days. If I'm in any sort of pain, it will almost always be because of pinched toes, not because of the high heel position. That pretty much only happens with pumps, which I have to wear pretty tight, or I'll walk right out of them. I always wondered how women can slip their shoes off easily and yet still walk normally. That doesn't happen for me.
The real secret to my success is how much I walk in heels. Almost every morning, I'm out there pounding the pavement in heels. I feel it really makes a difference. Standing in one spot is much harder than walking in heels (and I would argue in any sort of shoe), but walking gains you a lot of endurance as well as a sense of fluidity and grace if you give it long enough, and do it purposefully enough. Besides that, it's fun! To actually address the original question, I do not believe it's possible to "work through the pain" and gain much endurance doing incidental activities. Therefore, I don't hold it against anybody if they want to bring a pair of backup shoes just in case.