Low key is good, part of the photography is capturing the day as it unfolds I guess working quickly is part of being unobtrusive.
Don't be surprised even though she wants it low key that on the day you will be ordered around a bit at various points to get shots of her and the groom with various people
It is a really good experience as a photographer and you will find yourself buzzing by the end of the day.
Good move talking to her about it as well, you will then be able to construct a shooting plan and have something to work from.