Slow enough to feel the heel-to-toe sequence; fast enough to feel natural. Attention rests on the soles of the feet, or the lifting and placing of each step, or the whole body moving through space. Thoughts arise; one returns.
Zen kinhin, the Burmese slow walking of Theravadin retreats, and Thich Nhat Hanh‘s popularized walking meditation are variations on the same simple technology. Useful when sitting stops being useful, and always — as Thich Nhat Hanh taught — available on the way from anywhere to anywhere.