It is difficult to maintain quality -- and very very different from concert or academic type piano work.
Many of the suggestions above are excellent.
One I will really emphasize, though: develope for yourself a repertoire of music usable as preludes and postludes (and, if you church uses them, offertories). Be sure to include a few which could be used for special services, such as funerals and weddings. These don't have to be hard. They don't have to be spectacular. They don't even have to be long. They do need to be under your fingers, though.
Hymns are another matter. If you can't get your rector, priest, pastor etc. to give them to you in advance, you're not going to have much option but really working on your sight reading. Having been a church organist for many years, however, I might note that, when playing a hymn on the piano, there are a few tricks of the trade. First of all, keep going, whatever happens! If necessary, one finger on the soprano line -- almost always the melody -- but keep on going. Second, judicious use of the sustain pedal can help a lot. Third, you can prioritise the lines (someone suggested analysing the hymn for chord progression; not a bad idea): the most important line is the soprano; the bass is next, then the alto, and last the tenor. If you find yourself really challenged by a hymn -- and I assure you, we all have been! -- drop lines in reverse order (that is, drop tenor first, etc.)
Improvisation can be invaluable -- and you can improvise on almost any melody you want, in whatever classical style is comfortable for you (I have been known to improvise on "Twinkle twinkle little star"...).
If you are planning to be in the game for a longish time... a)seriously consider learning to play the organ; it's very different in some ways, but not all that hard to do and b)also seriously consider working with your rector, priest, whatever, to learn the liturgical year and how to fit hymns and other music to the scripture and season -- and take time to study your denomination's hymn book. Not just the music, but the words. With diligence, you may find that your rector etc. will begin to welcome your suggestions as to hymns, when he or she finds out that you know something about how they fit in (this is more applicable to the more orthodox churches -- Lutheran, Episcopal, Anglican, RC -- than to the more protestant churches as the former have a much more clear cut liturgical year).