A brief journey across the desert should have been the final step before entering the Promised Land. The geographical distance between Mount Sinai and the border of Israel stands in sharp contrast to the historical reality the Israelites actually experienced. Highlighting the time and distance serves as a powerful tool to show the sheer scale of the tragedy and missed opportunity that took place right at the edge of their destination.
The primary approach among commentators is that this detail serves as a sharp rebuke from Moses. The normal, natural travel time from Mount Sinai along the most direct route to the border of Israel is merely an eleven-day journey [רמב״ן, רא״ש]. The rebuke lies in the painful gap between the original plan and the harsh reality. God originally planned to bring the people into the land quickly and without delay [העמק דבר]. However, Moses points out that because of the sin of the spies, a simple eleven-day trip turned into forty years of aimless wandering [רשב״ם, ספורנו, צאינה וראינה].
Adding another layer to this missed opportunity, several commentators calculate the exact dates of the journey and conclude that God actually performed a miracle to shorten the trip even further. Out of a strong desire to bring the Israelites into the land quickly, He compressed what should have been an eleven-day walk into just three days. Yet, despite God's active intervention to speed up the process, the people's corrupt actions resulted in a delay of several decades [רש״י, רבנו בחיי, ספורנו]. Others note that the rebuke highlights how a direct path would have led to an immediate entry, but their sin forced them into a long, exhausting detour [בכור שור, ברכת אשר על התורה].
On the other hand, there are approaches that view the travel time not as a rebuke, but as a simple historical or factual record. One perspective suggests that the Israelites actually walked for exactly eleven days to reach the border, provided one subtracts the many days they spent resting at various camps along the way [רשב״ם]. Another viewpoint connects the timeframe to the teaching of the Torah itself. In this view, the eleven days represent the specific period during which Moses taught the people laws and Commandments as they traveled, right before the decree of wandering was handed down [אבן עזרא, ביאור יש״ר]. However, this idea is strongly disputed by others, who argue that the Israelites camped for much longer periods and that the vast majority of the Commandments were given directly at Mount Sinai [שד״ל, אם למקרא]. Finally, some understand the mention of the distance as a purely geographical marker. It simply pinpoints the exact location in the desert where Moses delivered his farewell address, noting that it was an eleven-day walk away from Mount Sinai [הכתב והקבלה].