I absolutely love walking meditation! First because it's incredibly relaxing and makes my head wonderfully clear. Second, as you already said, it's a great exercise to do before a sitting meditation. The latter, I've found is (there may be other factors, but this one I particularly noticed) because when you just sit down, two things need to happen: your mind needs to calm down, and you need to find a comfortable posture. The first happens automatically, but until you're there it hampers the second (that would probably lessen with practice--but is also highly dependent on mood etc). And sometimes by the time you're there (calm and comfortable), you're already sort of stiff and slightly cramped because you've been sitting for some time. If I do the walking meditation first, I don't get stiff cause I'm walking, and by the time I sit down, my mind's all calmed down and I can almost immediately find a comfortable posture (no twitches, no involuntarily tensed muscles you need to relax first) and can enjoy the sitting part of my meditation so much better (and sooner).
I should really discuss with my meditation-group to do such a type of warm-up more often, actually :)