How should a pastor decide what to preach?
I have no rules laid down for how it must be done. I certainly wouldn't have begun my pastorate with eight years in Romans. Romans was daunting to me in and of itself, and doing it at the start of my pastorate would have locked me into too great a project at the start when there were a lot of other things that needed to be addressed in those days.
A pastor should seek the Lord for the right combination between topical series (i.e. on love, death, money, etc.) and expositional book studies. Don't get me wrong: all sermons should be expositional. When I say "topical" I don't mean non-expositional. I mean that you take a text and you expound its message regarding money, and then you take another money-related text and expound it, etc. They're all textually based and expositional.
All preaching is expository in that sense, and so it should be. But working your way systematically through a book is one kind of exposition, and I think it should be mingled in with others. Because if you only do topical studies then you will never bump into the full counsel of God that needs to be exposed to the people.
If you preach right through a text then you can't get away with skipping passages. Skipping passages opens the door for people to accuse you of being short-shrift with a certain topic or not believing that all of Scripture is inspired by God. Or they'll think that you just really like to ride your hobby horse.
Therefore I think that mingling the two together–addressing topics that your people need to have addressed directly, and then doing book studies as well–is a great combination.
As regards the pace, I preached Job in five sermons. Then I preached Romans in two hundred sermons. You can do it both ways. You can take larger blocks and teach single sermons on their main points–even though you know you could easily preach ten sermons on each paragraph–or you can slow down and do a very in-depth study of each verse.
Try to discern what your people need and where they are in their understanding. And mix it up.