There's a reason people tend to pace while talking on the phone. I believe that there's a connection between kinetic movement and our ability to focus and think. I certainly feel that way for myself. Listening to audiobooks while I walk my dog has been a great way for me to be able to enjoy more "reading" during my day.
That being said, I would definitely advise against reading a paper book/ipad while walking outside, only because I'd be worried for your safety! But the treadmill on a walking speed sounds like a decent idea.