Aside from the many, many other benefits you'll get from making it a regular habit, it can really help to strengthen the muscles needed to maintain good posture (just be sure to walk with your head held up).

The beauty of walking is that it is so easy to work into your daily life. In the right environment it's a very pleasant thing to do. I often find myself heading out to get lunch and instead of stopping at that place a block away, I'll go an extra couple of blocks just because I'm enjoying the walk (usually, this has something to do with the trains of thought that walking encourages). It never feels in the slightest like 'exercising'. As a programmer, I consider walking while thinking to be one of my most productive activities.

I'm not suggesting that walking is the solution for you, but I'm confident that it will help to improve your posture if you don't already do a lot of it.