Genèse 26:1

And there was a mighty famine in the land of Kenaan, besides the former famine which had been in the days of Abraham; and Izhak went to Abimelek king of the Philistaee at Gerar.

BESIDE THE FIRST FAMINE THAT WAS IN THE DAYS OF ABRAHAM. Perhaps there was no famine in the world until the days of Abraham. This is why Scripture counts from it, for otherwise, what need is there to mention it? In my opinion the correct reason why Scripture mentions it is to tell us that people remembered the first famine, mentioning that on account of it Abraham went down to Egypt and there G-d did him great honor. It was for this reason that Isaac wanted to go in his father’s footsteps by descending into Egypt until it was said to him, Go not down into Egypt. The reason for the prohibition has been stated by our Rabbis: “You [Isaac] are a perfect burnt-offering and residence outside of the Land of Israel does not befit you.”
In my opinion, there is also included in this subject a reference to the future. Abraham’s exile into Egypt on account of the famine is an allusion to the exile of his children there. His going to Abimelech however was not an exile for he resided there of his own volition. But Isaac’s going [to the land of Abimelech, as recorded here in this verse, And Isaac went unto Abimelech] on account of the famine, does allude to an exile since he left his place against his will and went to another land. Now Isaac’s exile was from his own place to the land of the Philistines, which was the land in which his father had resided. This alludes to the Babylonian Exile, which took place in the land in which their ancestors had resided, namely, Ur of the Chaldees. (11:28), states his opinion at length that Abraham’s birthplace was not Ur of the Chaldees, it is necessary to say that the author’s reference here is to the time when Terah, his father, took him there, and while being there his life was saved by a miracle. See Ramban there at the end of Verse 28.
Know further that this Babylonian Exile mentioned is mirrored in the events which befell Isaac in that they did not take his wife in the land of the Philistines. Rather, his lot there was only exile and fear. At the beginning Abimelech said, He that toucheth this man or his wife shall surely be put to death. Later he regretted it and said, Go from us. Afterwards, he returned and made a covenant with Isaac Similarly, in the Babylonian Exile, they were exiled there because of the burning heat of famine, and while there, they were neither subjugated, nor were they treated harshly. On the contrary, their leaders were princes in the government. Later on, they said, Whosoever there is among you of all His people — his G-d be with him — let him go up, even alerting the princes and governors beyond the River to help them. Later on, they ceased work [on the House of G-d in Jerusalem] , and it ceased “for a season and a time.” Later, they again changed their policy and gave permission for the construction of the House of G-d, saying, That they may offer sacrifices of sweet savor unto the G-d of heaven, and pray for the life of the king and his sons.

מלבד הרעב הראשון אשר היה בימי אברהם, as we read in Genesis 12,10. There was no need for this phrase except to inform us that just as his father Avraham had traveled to Egypt on account of a famine, so now his son was about to emulate his father and to travel to Egypt by way of the land of the Philistines. This was the shortest route to Egypt, as we know from Exodus 13,17. G’d told him not to go to Egypt as he knew what his plans were.

אשר היה בימי אברהם, ever since that time there had not been as severe a famine in the Land Of Canaan as there was at the time the Torah speaks of here, a famine severe enough to cause the inhabitants of the land to migrate in search of food.

וילך יצחק אל אבימלך, in order to obtain a permit to stay in his country. This teaches us that Yitzchok possessed good manners, seeing he could have claimed this as a right under the treaty between Avraham and Avimelech.

ויהי רעב בארץ, in the land of Canaan.

מלבד, meaning that since the last mentioned famine in Genesis chapter 12, there had not been another famine in the land of Canaan. The only reason Avraham had left the land of Canaan at that time had been the famine, and his desire to save the vast herds of cattle and sheep owned by him at that time.

וילך יצחק, Yitzchok, relying on the treaty between his father and Avimelech the King of the Land of the Philistines, considered it the natural thing to do to move to the region ruled over by Avimelech in order to sojourn there until the famine would pass. Avimelech at that time had stipulated that the treaty between them would remain in force for grandchildren and great grandchildren of the signatories. (21,23)