Jesus becomes the divinely chosen precursor [or forerunner] of the turn of the age. Moses, Elijah, and even God are not the only signs for the alert that God's reign are coming.
Since this story so emphasizes the dazzling glow of the Jesus, Moses, and Elijah that we also need to pay attention to glory. The best known earlier example of a dazzling face transformed by being in the presence of God is that of Moses (Exodus 34:2, 29-35). After Moses has been in conversation with God about the future life of God's people, he descends from the mountain so reflecting the light of God's glory that he must cover his face lest he frighten the people. There are surely similarities to Jesus as He seeks to form a new people of God, has climbed a mountain, and is in conversation with God. Also important, in Daniel the "Son of Man" is also dazzling white. The mysterious messianic figure that will bring about God's will and God's justice is a supernaturally stunning figure (Daniel 7:9-14). As Jesus is transfigured Peter, James, and John and Mark's audience catches a glimpse of His reality as Son of Man, God's chosen messenger of the God's reign.
Putting all this together, we have a story that reassures Jesus' core disciples and Mark's readers: Jesus' predictions of betrayal, death, and resurrection are to be trusted. The struggles yet to come for Jesus in Lent should in no way diminish confidence in His promises or His predictions of resurrection. As Mark's gospel drives toward the bitterness of the passion and the ambiguity of an ending without a resurrection appearance, this story itself shines as a beacon of hope.