I want to continue the momentum, but I also have a timeline to keep and several groups of characters (seven groups, to be precise, with an eighth in pursuit and a ninth advancing to attack the eighth) to move toward the same place geographically.
There are two significant places left for characters to gather, and I've decided that the first of those places--the foothills between a rugged mountain range and a dense forest--will open Part Three. The second place--a ruined fortress--will end Part Four.
Place and time together provide an organic outline for the story. A decision and demi-climax will occur in the foothills; as a result, pursuit ensues, and the big finish occur in the ruins.
Needless to say, I've done a lot of thinking and played a lot of solitaire, moving cards around like moving scenes, pieces of a puzzle.