The feet called beautiful are the symbol of God's creative power in motion, the same power that is your own imagination acting. Feet carry a man, and here they carry the news that peace and every good thing are already yours to assume. Being sent means the power comes from within, not from an outside authority granting permission; the one who speaks the wish fulfilled speaks from the only self there is, God as I am in you.
Glad tidings of good things is the announcement, made first in your own mind, that the desired state is already so. You cannot bring that news to yourself until you have been told it, which is why someone must say plainly that imagination creates. Once heard and believed, the message walks. The good report you carry inwardly about your own life is the gospel, and your feet are lovely the moment you assume the end already reached.