It is all about P.O.V.
You don't have to worry about too much "side material" if you reveal things to your readers as the PCs learn about them. I have a very simple way of handling that stuff, if the PCs don't know the readers don't know - want to go into the details of the plot and the various factions and facets of the campaign? Use PC and PC/NPC dialogue to go over it instead of dryly reciting it for the reader.
This has the added benefit of helping establish and reinforce characterizations of PC/NPCs by giving their view of what it going on in their voice. It might help that the PCs in my game disagree a whole lot.
I also use footnotes to deciminate campaign setting info, that way it does not slow down the actual telling of teh story and only those readers who are really that invested in the game and the setting will take the time to read them. I also use the footnotes as way to make "out of game" comments on the mechanics of scenes and encounters. I don't know how "successful" these footnotes are (you'd have to ask Look_A_Unicorn or Jon Potter or one of my other regular readers), but they definitely make things easier for me, and if the notes are repetitive, well it does not matter so much.
But yeah, I guess writing is harder than running, but I find prep for running harder than both.