דברים לא:א,ב - וַיֵּלֶךְ מֹשֶׁה וַיְדַבֵּר אֶת-הַדְּבָרִים הָאֵלֶּה אֶל-כָּל-יִשְׂרָאֵל. וַיֹּאמֶר אֲלֵהֶם בֶּן-מֵאָה וְעֶשְׂרִים שָׁנָה אָנֹכִי הַיּוֹם לֹא-אוּכַל עוֹד לָצֵאת וְלָבוֹא וַה' אָמַר אֵלַי לֹא תַעֲבֹר אֶת-הַיַּרְדֵּן הַזֶּה.
דברים לד:ז - וּמֹשֶׁה בֶּן-מֵאָה וְעֶשְׂרִים שָׁנָה בְּמֹתוֹ לֹא-כָהֲתָה עֵינוֹ וְלֹא-נָס לֵחֹה.
Devarim 31:2 records that Moshe told the people that he was no longer able to come and go, and that G-d had told him that he could not cross over the Jordan River to get to the land of Israel. The beginning of this verse, “I am no longer able to come and go” indicates that there was a deterioration in Moshe's physical abilities. However, Devarim 34:7 records that "his eyes were undimmed and his vigor unabated" (JPS translation in Tigay, 1996, p. 338), which seems to imply that there was no diminishment in Moshe's physical condition.Rashi (on 31:2) suggests two solutions to this question. One, the reason Moshe could not go about was because of G-d's command, which is referred to in the second half of 31:2, but really he was completely fit. This is problematic since the second half of 31:2 appears to be an additional reason why Moshe was not able to be the leader of the people. Rashi's second suggestion is that the phrase coming and going in 31:2 refers to Moshe's ability to learn and expound the Torah and not his physical abilities. Yet, the phrase coming and going seems to refer to physical abilities, and it is difficult to claim that Moshe was no longer able to expound the Torah since he still has various messages to give to the people, from 31:3 to the end of the Torah.
Ibn Ezra (on 31:2) suggests that the coming and going in 31:2 refers to Moshe's ability to lead the people in war, which required extraordinary physical strength. He claims that 31:3, which refers to the upcoming conquest of the land of Israel, is a proof to this idea. However, the point of 31:3 is that G-d will take care of the fighting and then Moshe would not need to have great physical strength. Furthermore, Moshe was not going to be the general going out to battle. Even by the war with Amalek, forty years earlier, Moshe did not fight Amalek physically, Shemot 17:9. In addition, the people had just had several wars with Sihon, Og and Midyan (Bemidbar 21:35, 31:3-6) without Moshe having to be directly involved in the war. Also, Luzzatto (on 31:2) notes that the phrase "coming and going" in 31:2 just refers to going about and not to leading the people in battle.
The Ramban (on 31:2) writes that Moshe was completely fit, and his statement that he could not come and go was just to comfort the people about his upcoming death. The idea is that Moshe was not being completely honest with people that he was telling them that he was not well, even though really he was feeling fine, in order that they would accept his death. This seems very unlikely.
The Abarbanel (1999, pp. 489,490) suggests that Moshe was telling the people that shortly in the future he would be losing his strength. This is problematic because 31:2 seems to mean that at the moment when Moshe was speaking his strength was already beginning to ebb.
A simple answer is to re-examine the meaning of 34:7. Does 34:7 state that Moshe's vigor was unabated? Tigay (1996, p. 338) points out, based on the Ibn Ezra's comment on 34:7, that 34:7 means that Moshe's skin had not become wrinkled. This means that Moshe showed no signs of aging, but many times a person can look fine but still not feel well. Thus, 31:2 could just mean that Moshe was telling the people that even though he looked young and fit, still he knew that his physical strength was ebbing and consequently he no longer had the physical strength to be their leader. In fact, because he looked young, he had to tell the people that really he was not completely fit.
This deterioration might have been in some ways a comfort to Moshe that he would accept his impending death. If he had the vigor of a twenty year old, then it might have been harder for him to accept that it was time to die, while if he understood that he was aging, then it could have made it easier for him to accept that he was about to die.