Rashi explains that Moshe was not suggesting that the frailty of old age impeded him from going out and coming in. The Torah itself later attests to his remarkable vigorous health and physical vibrancy. Rather, Moshe indicated that Hashem had forbidden him from continuing in his current position as leader. Rashi’s explanation does not seem consistent with the literal definition of the word kfut which means “able”. The literal definition would be translated as, “I am no longer able.” This translation implies physical impediment and inability to continue.
We may suggest that, indeed, when a tzaddik is prohibited from doing a specific activity, he literally becomes incapable of carrying out the particular endeavor. Every Jew should strive to attain this spiritual attitude towards mitzvos. We should be so totally devoted to Hashem’s essence that we are simply unable to do anything which is contradictory to Torah dictate.
This author once heard from Horav Boruch Sorotzkin z.l. that he simply could not oversleep Shacharis, because he had to daven with minyan. This was the Rosh Hayeshiva’s attitude in all areas of avodas Hashem. If there was a mitzvah to perform, he simply was not tired or sick, even when his body was invaded with disease and racked with pain. This was due to his ability to transcend the physical realm with spiritual strength.
The Ibn Ezra explains the command of “you shall not covet,” in a similar manner. One should view his friend’s property/object/wife, as being unattainable and out of reach. He uses the analogy of a peasant in relationship to the king’s daughter. The peasant is clearly aware of the impossibility of his marriage to royalty ever taking place. Because he recognizes the impossibility of such a union, he harbors no desire whatsoever for her. This should be our attitude towards property which is not ours. The Torah prohibits it, and therefore, we are unable to have it. Thus, our desire for it is totally precluded.
