If you reach out your hand to touch someone next to you. Is it you touching the other person or your perception of the sense of your own hand that you feel?
Everything you feel, see, hear, smell, touch is within you. it is not?
What you consciously experience is within you. What I've read is that when you are awake, experiencing a sensory perception involves many different areas of the brain. Vs when one is asleep, that sensory perception goes directly to the area of the brain with is involved with that sense.
When we perceive something, it triggers memories. Those memories trigger all the different parts of the brain involved in those memories. So like when you smell bacon, it triggers all of those experiences recorded in your memory associated with bacon, like a visual representation, sound, taste, etc...
So it's not just the actually perception alone that we experience. While the majority of what we experience is "within", it doesn't mean that it's all in our head. Something external stimulates your nervous system to get things started in most cases.
I suppose folks could be "daydreaming", somewhat oblivious to what is going on around them.
Walking meditation in my opinion is about being aware of external stimulus while shutting down as much as possible the internal "noise" that accompanies the majority of what we experience.