Well, you & I have discussed burnout before, and you know what I went through recently myself---what would you advise ME to do in this situation?
Follow your bliss.........as flattering as it is to be asked, if you don't want this position, don't take it. If your boss is as great as she seems to be, she'll understand, and she'll find someone else. You've been through too much and grown too much wisdom to allow yourself to be placed in a situation where you KNOW you're still vulnerable.
Now me, I'm
thriving on the 9-4, M-F routine, and I'm putting in more time than that, but it's MY CHOICE to do so---
I decide the hours and the pace, I take breaks when I feel like it, and if I want to go in on a nice quiet Saturday or Sunday and take a day off during the week, I can do that too. I have my own office with a door that closes (although it's open pretty much all the time unless I'm doing something that requires intensive concentration---or surfing allnurses!

) AND I can be in there---or not---as I choose.
The key here is control: I do my job the way
I want and need to do it. Therefore, there is no stress in working long hours, or in being there however many days a week I feel I need to be. Now, IF you did want to try the management position, and IF you could work out a similar arrangement with your supervisor, you'd do OK. It's all about having choices and being trusted to do your job your way.