Ye Olde GM
One of the things I loved about Encounters (and to an extent, 4e in general) was only having to plan for the next encounter that week. Not sure how well that philosophy holds up now, but essentially you should still concentrate only on your next session. Keep your larger plans loose and flexible. And build your toolbox of maps, encounters, NPCs, and whatever else you use that can be utilized in a pinch when things turn left instead of right.