Or HaChaim on Genesis 26:3/אור החיים על בראשית כ״ו:ג׳

1 כי לך ולזרעך אתן, "for I shall give to you and to your descendants, etc." The land of the Philistines was included in the this gift of the land. Nachmanides also understands our verse in that sense. He writes that the reason G'd said "to you" i.e. not only to Isaac's descendants, may have accounted for the phenomenal success Isaac experienced when he planted in the land of the Philistines (26,12). G'd also intended to tell Isaac that he would be farming successfully there. [after all, Abraham, Isaac and Jacob were shepherds not tillers of the soil. Ed.] Perhaps it was because Abraham at the time had failed to establish himself on the soil of the land of the Philistines that G'd told Isaac that he would stake a better claim to that land by ploughing and sowing it. This is why G'd was specific in speaking about כל הארצות האל, "all of these lands."

2G'd also wanted to assure Isaac that Ishmael would not share in any of the promises made by Him to Abraham. This is why G'd stressed the word לך, "to you." G'd added the word ולזרעך, "and to your seed," to exclude children of Isaac who would not qualify as his seed in the spiritual sense of the word. This is similar to G'd having told Abraham (21,12) "through Isaac will your seed be known." Nedarim 31 derives from that wording that not all of Isaac's seed would be worthy.