How Moshe Rabbeinu maintained his strength

Moshe Rabbeinu states that he has reached 120 years without any diminution of his strength and capabilities. However, on the last day of his life he suddenly states that he cannot continue to "go out and come in". Rav Shamshon Rafael Hirsch states that this shows that it was his position as leader that caused him to lose his strength so suddenly. While he was the leader, Hashem helped him maintain the strength to continue his duties. As soon as Yehoshua became the leader, the extra strength was transferred to him and Moshe was left only with the natural strength of his physical age.

We see this in the strength that many of the gedolei hador exhibit as they continue to serve Bnei Yisroel. The strength that they exhibit is given to them to serve Bnei Yisroel and is a measure of the hashgacha that Hashem shows to His people.