This land is Gerar.
Rabbeinu Bachya writes:
ויזרע יצחק בארץ ההיא, “Yitzchak planted in that land, etc.” The reason the Torah emphasised “in that land,” is that it was hard ground, did not usually produce much. Considering the fact that the year was one of famine the achievement by Yitzchak in securing a hundredfold return on his seed was all the more remarkable. The place where he had planted the seed was Gerar. This is where these remarkable “hidden” miracles happened for him. The land was so parched that when the local people finally found some water, they exclaimed: “we have found water!” (verse 32)
As far as how he came to own it. His father Avraham had it first and was given over to him by Hashem. The Sifrei (Devarim 8:1) notes:
(Devarim 1:8) "which the L-rd swore to your fathers": Why need it be mentioned (in addition to this) "to Abraham, to Isaac, and to Jacob"? (To stress that) each one was worthy of this in himself. An analogy: A king gives his servant one field as a gift, as it is. The servant goes and improves it, not leaving it as it is, but planting a vineyard and bequeaths it to his son — who does not leave it as it is, but improves it and bequeaths it to his son — who does not leave it as it is, but improves it. Thus, when the Holy One Blessed be He gave the land to our father Abraham, He gave it to him only as it was, viz. (Bereishis 13:17) "Arise, walk in the land in its length and its breadth, for to you shall I give it." Abraham arose and improved it, viz. (Ibid. 21:33) "And he planted a fruit grove in Beersheva." Isaac arose and improved it, viz. (Ibid. 26:12) "And Isaac sowed in that land and he found in that year a hundredfold." Jacob arose and improved it, viz. (Ibid. 33:19) "And he bought the part of the field where he had pitched his tent." If so, why need "to your fathers" be written? If in respect to the oath to the fathers, this is already written. This refers to the covenant with the tribes, viz. (Habakkuk 3:9) "the oaths to the tribes, an enduring word."