It's easy to look at some people and assume that everything is easy, that they don't have scars. Zechariah and Elizabeth were righteous people, walking blamelessly in all the commandments and statutes of the Lord. But they carried a deep wound, as they were aged and had no children. This was a hard thing for them.
It's a reminder for us that even the strongest Christians carry wounds, even if we cannot see them. Our faith depends not on our strength, thankfully, but on the strength of Christ. This is why the Apostle Paul was able to say that even in his weakness, he found strength. He had wounds, just as Zechariah and Elizabeth did, just as you and I do, and he looked to Christ for his strength.
God is at work in their lives, just as God is at work in our lives, despite our wounds and fears and weakness. That is not an obstacle for God.