This is one of those mysteries that you can ponder for a long time and never quite finish pondering it. The “preaching of the kingdom” covers just about everything in the gospels: the Sermon on the Mount, the parables, the cures, the Loaves and Fishes, the Bread of Life discourse–all of Jesus’ preaching and activities. In that sense, it’s an easy mystery to pray, because you can pick your favorite image or story or event and meditate on that.
But I think it’s also fruitful to use this to ponder our own sense of mission. For nuns and sisters, it’s their prayer and their apostolate that is their preaching. For married women, it’s their married life, and growing in holiness with their husbands; for a mother, it’s taking care of her family, and raising her kids to know God and to be devout Catholics. For the single, it can be a little harder, but we are all called to holiness, to prayer, and to bringing that to the world.