I am working on something loosely in this genre. I have also read a great number of works in this genre, and what I find is that all to often, for my tastes at least, the main idea, be it a plague or whatnot, is just an excuse for seemingly endless battle/fight or chase scenes. I've also noticed this about dystopian themes. Some of them are very well done and I enjoyed a few, but it still seems like it is almost obligatory to have some sort of "enemy" that either attacks the protagonist(s) or is attacked, or both, usually for the length of the book. My own concept does not lend itself to this device, though I am not above using some sort of antagonist(s) in order to add some diversion from the main theme of the survivor(s) coping with the situation If I have a question at all, I suppose it is whether anybody else has noticed the excuse-for-battle-and-chase-scenes bit.