if you've got 20-30 minutes to practice, you might want to find some exercises that allow you to 'multitask' - that is, work on technique issues while also picking up some new theory ideas.
For playing in church, the most important area of your playing to develop is your ability to interpret chord charts in the common styles used in your church. Find a book that teaches you about the basics of chord construction - Major, Minor, etc, and shows you how they fit together within a specific key.
Ed Friedland probably has a book that covers these issues, and from my experience with his other teaching materials, he's one of the best and clearest tuitional book writers around - check out www.edfriedland.com
and see what's on offer.
Remember to take it slow, work on your sound and the specifics of your timing as well as getting the notes in the right order, and try to contextualise any exercise as soon as possible within a 'real' musical example - find a song that incorporates that principle. If you can't find one anywhere, it may be that the exercise you're studying isn't that important right now. Lay it on one side and work on something you're going to use next Sunday morning...