I think 40 would be too tight for backpacking generally...if I was going with one lens, I wouldn't want any tighter than 28mm, but would also have a backup for wider. (somewhere in the 15-20mm range) If you will be backpacking with a group and need to stay together, then it may not be practical for you to take 12-16 photos for stitching to capture a wide scene. If you group likes a lot of breaks and is willing to stop for you, then it might be ok.
Pretty much every time I go backpacking, I've got my 16-50mm DA* lens. I trust it out there due to the weather sealing, and it opens up wide enough for my liking, and at the same time can still do pretty good portraits and tight landscapes. I have taken a 28mm prime lens as a second lens to keep on the camera during good weather when wide isn't needed, it's great for general people shots, groups shots, most scenery. I have also taken my 50-135mm DA* as a second lens sometimes. Definitely not a necessity, but I did use it for a few shots that I could not have managed otherwise. However, K-7 + 2 DA* lenses + tripod eats up a good 7 or 8 lbs.
Anyways, back to your original question, if I were you, I'd definitely take the 16-45mm, and keep it on the camera. Throw the 40mm in your pocket if you need to make a portrait or have a critical landscape that needs every pixel of detail resolved. Actually I would just say leave it at home, but I know you want to take it
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Whatever you end up choosing, please let us know and then share you experience and some images! I always love to hear how people feel about their lens choices for traveling (especially backpacking), especially after they get back to know if there are any shots they absolutely loved or missed due to inadequate lens selections.