Who looks more psychotic, people singing along with their iPods or people with those Borg-like Bluetooth earphones who look as though they're talking to themselves? If it's people singing along with their iPods, I'm in big trouble.
Speaking of the Borg talkers, a couple of years ago I was walking in London and saw a guy in front of me talking and gesticulating wildly. I thought he was on the phone, but when I got ahead of him, there was no phone there, and no consecutive thought process behind what he was saying, either.
I'm doing more running on the walk but am still no candidate for a marathon.
Playing music is better for fast walking/running than listening to audiobooks. Current audiobooks on my iPod: The Lion's Pride, on Theodore Roosevelt; The Omnivore's Dilemma, which was so depressing the other day as I was walking by a beautiful apple orchard and some fields that I switched away from it; A History of Rome and Team of Rivals, which I haven't started yet; and A Crack in the Edge of the World, about the San Francisco earthquake of 1906. I'm 4 hours into it and San Francisco hasn't been built yet. One disadvantage of audiobooks: no skimming.
Thoreau would be appalled to see all of us walking with iPods in our ears, but maybe he wasn't tortured with thoughts about all that he had left undone and the deadlines he was pushing against while he was walking, either. I know we're supposed to be contemplative and all that while walking, but the iPod helps to shut out those anxiety-producing thoughts, so what's the harm?