Dr. Rotwang has some thoughts on that:
Seriously. I'm just not that good at picturing landscapes. I'm terrible at it. They just don't...they don't occur to me, see? I feel like I should be thinking that this-or-that place in my game setting should look like this forest or that gorge, or what-have-you, but...I don't.
And it bugs me.
I've been trying harder lately, but I wonder if the culprit is unfamiliarity; in other words, that I don't get out much. Well, not into nature, anyhoo. I wonder if I don't think so quickly in terms of "nature looks like this", when I'm GMing, because I don't go look at it a lot.
I was lucky to have the chance to join a Boy Scout troop that went camping every month. I got to see a lot of nature, from sprawling dairy farms to cozy youth camps. I was even luckier to have cheap parents who insisted on camping rather than staying in motels when we drove long distances. (And the apple doesn't fall far from the tree lemmetellyou...)
And I was always thinking D&D. Maybe because we were always doing cool D&D things in scouts, like archery or map reading or hiking with packs. Some of the older guys ran wild, Monty Hault campaigns, with stuff cribbed from Gamma World and later Star Frontiers mixed in with their magic swords and staves of the magi. We didn't do that much when I got to be one of the older guys, but I remember sitting up late one night while the guy I was sharing my tent with and I read Dragonlance novels by flashlight.
So yeah, there was a lot of me thinking, "this is where the goblins live" and "this would make a great place for the evil baron's army to camp", and "yeah, ambush right here". The hard part, of course, is describing these scenes with enough energy and personality to make them really pop for your players. Nature can be so intense and broad in the amount of sensory data it just floods you with that it can be tempting to do the same to your players. Paring that information down, while putting them in that place is difficult. The keys, I think, are first to be brief. Five quick sentences at most. More than that, for me, and eyes begin to glaze over. Second, hit at least two senses. Honestly, for outdoor settings, you should probably be hitting four. Forests are full of smells and sounds, and they can shift every handful-dozen feet. Birdsong is common, and any wind makes the trees whisper to one another. Everything has a smell, from the loam underfoot to the gurgling stream dancing alongside the trail. You're always being touched by something. In the forest, the sunlight pierces the canopy of leaves in discrete shafts, patches of warm sliding across you as you walk. In the open, there's the sun, and sometimes the wind, either cooling relief in summer or biting assault in winter. And if you're really unlucky, there's the army of ants swarming up your leg when you stopped to rest in the wrong spot.