Deuteronomy 1:9 - The Popular Commentary by Paul E. Kretzmann

And I spake unto you at that time, saying, I am not able to bear you myself alone. Moses does not observe the chronological order of events, but selects such as were of importance for his present purpose. It was while the people were encamped at Horeb that Moses arranged for assistants, Exodus 18:13-26. The seventy elders were appointed at a later date, Numbers 11:14-30.