Marching refers to the organized, uniformed, steady and rhythmic walking forward, usually associated with military troops. Walking is.. well.. regular walking.^^

Do an image search for "marching", then another image search for "walking". The difference is very clear! :) https://images.google.com/

As others have noted, marching is a specific KIND of walking. Unlike some other languages, in English "march" is usually used literally, or close to literally. "March" always gives us a mental image of soldiers marching on parade. We have this image even if we just mean one person striding purposefully--"I'm going to march right into his office and demand a raise." We don't usually "march" just to mean "progress" or "move forward." There is always some idea of rapid, rhythmic, synchronized steps.

look: when u say other marching means u want them to walk ,but this walking is for being ready to start a war