Possibly, depends on a few specifics
What it sounds like you're doing is a style of railroading "Illusionism" (as pointed out by firedraco in the comments). This is essentially presenting a false choice in front of your players, when no real choice exists.
Why did I say possibly? Because there are a few specifics you can do when you run the game that make this method a time saving preparation tactic that doesn't invalidate choice.
For instance: If your players decide "Hey, that mansion we've found looks boring/scary/whatever, let's go down the other path," do you force them to stay at the mansion? Do you quickly swap fluff differences (Ok, the mansion is now a cave complex, the skeletons inside are now in the cave)? If the answer to these is yes, then you're definitely railroading.
On the other hand, if your players come upon the mansion and then they say "I don't want to go to the mansion" do you instead say "Ok guys, I need ten minutes to prepare the area down the other fork, you caught me with my pants down"? Then you aren't railroading, you're respecting their choice by allowing something meaningfully different to happen.
Blind Choice is no choice at all
That is to say; a choice between a road or a forest doesn't matter if there's no meaningful information about what lies upon the path or at the destination. Since it's a road or forest, you could easily say "Well, the road is well trafficked and speedy, but bandits hit there often. The forest is safer and has basically no risk of bandits, but there are weird creatures there..." Then you've provided the information to make said choice meaningful.
Oh yeah. Players get a sense for railroading when they're continually presented with blind choices, particularly blind choices that always lead to the next step in the "plot". They may not say anything because they're being polite, or they're enjoying the game anyway, or any other number of reasons. You aren't going to fool them, at least not for very long.
I highly recommend Courtney Campbell's series on the quantum ogre, the first one being here.
How to avoid railroading?
Well, some players don't actually mind railroading, at least not the nice kind that puts them on the plot where they feel involved and not the hamfisted kind that arbitrarily says "you can't do that". If your players are like this, you've really got not too much to worry about (except that they might expect you to provide all the life and effort, which will suck).
On to how to avoid railroading (Macro-Scale): Have an area map. Put a few cool things around the area, along the road, in the forest, whatever. When your group comes to the fork in the road, you'll know there's at least something along both paths to be interesting. Provide information about areas. A lake with an underwater city of evil frog men that invade the nearby villages can be as simple as a paragraph of description or less. Learning to be comfortable improvising off the bit of information linked to each area (and then adding that info to your notes) is one of the best things you can do to improve as a (non-railroady) DM. You don't need (or want) a prepared, defined plotline as a non-railroading DM, since plotlines are often easily destroyed or subverted by the whims of the players. This doesn't mean you can't have a loose plot with characters, just that it's going to get sent in wildly different directions when you let the players play and you'll have to not get attached to any specific direction.
(Mirco-scale): You say you include one puzzle and one combat encounter per area? Try making a small but non-linear dungeon instead. From The Alexandrian's Jaquaying the Dungeon:
The lines are linear paths that can be taken (including lines of rooms + hallways). Note how the paths loop in on themselves, allowing for backtracking, exploration, and the ability to skip stuff. If your players are curious and engaged, they'll appreciate this ability to skip around encounters and puzzles, and they might even backtrack and explore when they don't have to. Don't be surprised if they don't, either, but missing content is part of the risk of free choice. If you need a boss encounter to happen, just put it as a linear path at the back of the dungeon, that all optional paths will connect to.
None of this is comprehensive, but hopefully this helps you out and gets your mind spinning with ideas.