I learned this the hard way a couple of campaigns ago, you cannot ask the players what they do next and then spend 2+ hours waiting for an actual response. You will waste everyone's time. This is when your preparation comes into play. At that point LEAD them.
By the same token when the players are on the move, the best thing to do is get out of the way and "see with them" where it leads. This is where your preparation also pays off - improvisation is 90% preparation couched to look like you just thought that up.
That's great advice, yes (sorry can't XP!). You've identified both the dangers of the linear railroady GM, but also the purist sandboxer - the guy who insists it's 'not a real sandbox' if he ever has stuff happen that's not player-initiated. You show the good middle path - be ready to follow where the players lead, but also have plenty of stuff ready to run if they prefer a reactive stance.