"Ped" is from the Latin. "Pod" is from the Greek. For some reason, we use the Latin particle to mean walking and the Greek particle to mean the structure of the body part itself.
Othopedists study walking. Podiatrists study feet.
Pedestrians walk, and when you stop them from walking, you impede their progress.
It gets tricky when you get into animal names - Quadrupeds, Centipedes, and Millipedes are defined by their number of feet, true, but also the number of feet they use for walking, whereas arthropods, gastropods, and the like are defined by their feet, but in the structure of the foot itself, not in how they use it to get around.
Actually, a podometer might measure the size or quality of your foot rather than their number.
Even I get confused by "podium" and "pedestal," however.