What is the goal of your DMing? I mean, what is your ULTIMATE goal? From my experience, when you get to the end of a gaming session, if the PLAYERS had a good time, then you did your job. IF, on the other hand, your DMing so that YOU get the results you want, your players are going to resent that, and lose interest. If they like to plan, understand that, know it in advance, and make fewer encounters or situations for them to have to plan out.
As a DM you have means to compel the party, but you should use those sparingly, or in situations where you need the pressure of the situation to be obvious to the players. Typically, a group has someone who generally doesn't like to wait, and they'll pressure the party to action. If the group thoroughly enjoys that stuff... Then they're getting to do what they enjoy.
Now, are you implying that they're making it more difficult for you to challenge them because they're being so meticulous, then... Well, here's a couple of things to consider. What if they plan like that because they're trying to prevent YOU from screwing them? What if they beleive that every time they get into a situation, they can't succeed? What if they think that, no matter what, if they don't do this, then you'll punish them, or the bad guy will always choose or have chosen an action that defeats them? If they plan something that totally busts the encounter you had planned,... then that's a good job! Every now and then, it really empowers a player if they're spellcaster steps up and disintegrates the giant before the first blow lands!
So, the first thing I would do is to determine WHY they do this. If it's what they like, then you're good. If they do this because they're paranoid about your DMing... you might want to rethink the game.
I don't ever go into a session, as a DM, with a set milemarker, because the players will invariably exceed it, or never reach it due to their interest in something (oft-times) completely inane to the adventure. What do you do then?... What I do is use that tangent and explore it. If it sidetracks the adventure, but the players have fun, then it worked out. Flexibility as a DM goes a long way.