And the final chapter!
@yronimous: I feel I have a GM who can railroad us even more than the GM you described; it's not "You stand at the outside of the dungeon, what do you do?" "Wrong!" ...it's more like... "My GMPC is leading you into the dungeon as you follow behind." The story can keep going without us making any decisions. Anyway, as you explained, I hope to provide my players with enough information, set the mood of the campaign and let them make informed decisions which have meaningful consequences.
@Cap. Darling: I do plan on using some "All roads lead to Rome" tactics, especially with any number of NPCs able to drop the same plot hooks, and perhaps any number of subplots lead to a bigger plot.
@Devilkiller: There are certainly some skills I'll need to master: improvisation and, failing that, controlling the flow of the game to non-offensively stall the PCs, thereby buying you the time you need to create a dungeon/flesh out a plotline that the PCs will explore next time.
@Bodhizen: Good reminders; I intend to have a weather forecast for the entire campaign. Players can railroad is a concern, I know one player who can get a little dominating. I could use some in-game consequences and punish this player, but I'd sooner take him aside and discuss it with him. He's been in my Pathfinder group for 7 years, so I'm comfortable enough.
@SteveGeddes: I'm unsure how comfortable my players will be without rails, I'll do my best to guide them subtly throw in plot hooks and decide for themselves which they'll bite. I do have a player who, when the story isn't moving at a pace she enjoys, will shout "Find the Plot!" a parody of "Find the Path", hoping similar magical results will be reached. Hopefully when the PCs stumble on one of the multiple plotlines found in my sandbox, it won't feel aimless nor require further castings of "Find the Plot"
@MrCharisma: Hope you don't mind if I straight up steal some of what you posted for my campaign. Great ideas. Really digging the separate yet interconnected and constantly moving plotlines.
...and it's over! Thanks again so much to everyone who posted. ...and feel free to post more. This has really helped inspire me to keep on plugging away at building my sandbox.