Moses at 80 felt completely insufficient for the task. Hopefully the older we get, the more humble we become. When we are young we think we have all the answers. When we get older, we realize we don't even know all the questions. Young seminary students think they understand the Bible.
Older pastors realize they'll never fully understand even a small portion of Scripture.
I returned at 21 amazed at what the old man had learned in three years . Moses' response was just the humility of his years. He did not feel up to the task. Moses sinned in his pride, in running ahead of God and in murdering an Egyptian. As a result, he faced the consequence of 40 years in the desert.