In most churches that I have been involved with, the hymns have been chosen by the Priest, Lay Reader or some other designated person rather than the Organist. Hymns also tend to be chosen to fit in with particular readings or themes which the Organist may not always know about in advance.
If you are lucky you will have plenty of notice of what is required, (our hymns are generally chosen a season at a time), but if they are less well organised you may not get to find out what you are playing until a day or two before services. Being a quick learner and having a wide repertoire is therefore important.

I don't know what sort of congregation you will have, but many older congregations don't like change, (especially if it means singing more quickly than they are used to), so it is good to be adaptable.
I would also suggest that you attend their Sunday morning service this week if at all possible to see how they do things, how well organised they are, how they like to sing, what sort of music they like, etc, etc.
This may sound like a silly question, but I assume you have past experience of leading hymn singing?
Good luck!
SB