How is walking meditation different than other forms of activity? I've read that things like aikido, yoga, swimming, running are not meditation. How is walking meditation different?
I've read that things like aikido, yoga, swimming, running are not
  meditation. How is walking meditation different?


Could you cite the source or teacher who says this?

In my experiential understanding, and in general, in Buddhist philosophy, any action done mindfully is meditation. The goal of Buddhism is to be mindful of our actions all the time.

The Buddha goes so far as to say that it is not merely by reciting verses in praise of awakening, that we pay homage to the Buddha, but every moment we are mindful, we pay homage to the Buddha within us.

One can certainly practice running meditation if one's mindfulness is very strong. 

Walking meditation is safe for the beginner who can observe the action of raaaaiiiissssssing his feet, and loooowwwwwweeeeering his feet, in many dimensions - the tug of the muscles, the raising of the hem of his garment, the brush of wind under the foot, the weight of the sandal as it hangs loose and so on.

I imagine if one was learning to observe like this in detail with an activity like swimming where rapid strokes are required, it can lead to a dangerous situation. As it is, it is common to see people unused to walking meditation stumble on their first attempt because they are so lost in observation, they forget to balance.

In Akido, and other martial arts, Mushin is a prized quality of mind.