For generations, a number borrowed from a Japanese marketing campaign has quietly governed how millions of people measure their daily movement. Now, Australian researchers studying more than 64,000 British adults have found that the body is more forgiving and more flexible than that single figure suggests — that a brisk 4,000-step walk and a leisurely 7,000-step stroll can offer the same protection against early death, up to 43 percent lower risk, so long as the movement becomes a habit rather than a performance.