I thought about talking about the way the priests were chosen to burn the incense.  The priests were divided into 24 divisions.  Each division had about 1000 priests in it.  Each division came to minister at the temple for two weeks out of the year, along with the Feast of Pentecost, the Feast of Tabernacles and the Feast of Passover.  Zechariahs' division was serving at the temple and chose who would burn the incense by lot.  If each priest served on a rotation, they would burn incense once every 14 years or so!  Life expectancy was about 50 years at that time (for those who made it into adulthood - 3 out of 4 children born died before the age of 10)  If you figure Zechariah was about 20 when he became a priest, he would have had maybe 2 opportunities in his lifetime to burn incense!