For you to learn ASL, yes, it could take years. The 3rd or fourth were way easier than the second. For you to learn how to interpret, that's a whole other story. Interpreters for the Deaf is one of those occupations where its advised one carry malpractice insurance. Actually, if you really know sign, interpreting for the Deaf in a religious situation is probably one of the easiest jobs to do.
Religious services tend to be lots of same old same old.
You already know all the prayers and songs. Slide them into sign. Have the minister e-mail you a copy of the sermon a day or 2 in advance so you can look up the big words and get clarification to true meaning. My personal opinion on the subject is unless there's a lot of other Deaf or people who know sign in the congregation, why bother. There would be no community.