How Walking meditation happen Without any thought automatically start the walking and it's last for 15 minutes. How it's related to meditation.
There are four meditation postures: walking, standing, sitting and lying down. Walking is beneficial:

AN5.29:1.1: “Mendicants, there are five benefits of walking meditation.
AN5.29:1.3: You get fit for traveling, fit for striving in meditation, and healthy. What’s eaten, drunk, chewed, and tasted is properly digested. And immersion gained while walking lasts long.

And here are some challenges to overcome while walking:

AN4.11:1.1: “Mendicants, suppose a mendicant has a sensual, malicious, or cruel thought while walking.
AN4.11:1.2: They tolerate it and don’t give it up, get rid of it, eliminate it, and obliterate it.
AN4.11:1.3: Such a mendicant is said to be ‘not keen or prudent, always lazy, and lacking energy’ when walking.

Here are the details of walking:

MN91:9.34: These are the thirty-two marks of a great man possessed by Master Gotama.
MN91:10.1: When he’s walking he takes the first step with the right foot.
MN91:10.2: He doesn’t lift his foot too far or place it too near.
MN91:10.3: He doesn’t walk too slow or too fast.
MN91:10.4: He walks without knocking his knees or ankles together.
MN91:10.5: When he’s walking he keeps his thighs neither too straight nor too bent, neither too tight nor too loose.
MN91:10.6: When he walks, only the lower half of his body moves, and he walks effortlessly.
MN91:10.7: When he turns to look he does so with the whole body.
MN91:10.8: He doesn’t look directly up or down.
MN91:10.9: He doesn’t look all around while walking, but focuses a plough’s length in front. Beyond that he has unhindered knowledge and vision.