to walk: to move about on foot, and from that, to carry on one's life in a settled pattern of conduct
The plain sense is bodily: to be up and moving on foot, to go about from place to place. In the Gospels that is almost always what the word does. Jesus walks by the sea and along the roads, a healed man takes his first steps, and on two famous occasions someone walks on water. The word pictures ordinary, visible motion.
From that picture the New Testament draws a second, deeper use that it inherits from the Hebrew Scriptures, where to walk in God's ways means to live by them. Paul especially turns the everyday verb into a way of describing the whole shape of a person's life: how you walk is how you live. So believers are told to walk in love, walk in the light, walk by the Spirit, and walk in a manner worthy of their calling, while the opposite is to walk after the flesh, to walk in darkness, to walk as the disordered do.
The two uses are one word because they are one idea seen at two depths. A life is a long road, and the steps you take day after day are the conduct you build. English keeps the same metaphor when it speaks of someone's walk with God or of walking a hard road, which is why translators can often keep the literal word and let the figure carry.