It's not that hard if you can avoid being eaten or crushed by your environment
The puzzles are all well-constructed, but some old-fashioned exploring can help you solve most of them.
And, of course, intelligent application of physics
A simple wooden ramp can help a lot occasionally.
Oh, and don't panic!
(I admit I had to stop playing for some time to calm down after I encountered the first diseased dog...)
Concerning the walkthrough, this forum section seems to serve that purpose quite well. After all, a walkthrough should always be your last resort, for those situations when you're really stuck in the game.
When I completed the game, I might start compiling one though, and post it here when I'm done with it.
But seriously, try it without a walkthrough first. When you're reading along with your progress, a lot of Penumbra's atmospheric awesomeness is lost, since you always know what will happen next, and what's lurking around the corner.