When will you delight in Hashem?
Radak: If you will make Shabbos a delight, you will delight in Hashem. I.e. He will put on you influence of good until you will delight in Him and thank Him and His goodness, for everything is from Him. This is delight of the Nefesh.
Radak citing Rav Sadya Gaon: Your bodily delight will be in Hashem, and unlike fools, about whom it says "Lo Navah li'Chsil Ta'anug" (Mishlei 19:10). A wise person will not over-indulge, rather, the proper amount, for he engages in Chachmah; he delights more in it. Bodily delights in the proper amount benefit the intellect and strengthen its three powers - memory, discernment and thought.
Malbim: The delight that you will find will be a spiritual, Divine delight of the Nefesh.
On what high places will you ride?
Radak: This is like "Yarkivehu Al Bamasi Aretz" (Devarim 32:13), i.e. Eretz Yisrael, which is higher than all lands. I.e. if you will be exiled from it, He will return you to rule over it in the reward of guarding Shabbos and other Mitzvos. This promise is for all Bnei Galus. If it is close to the time of Ge'ulah, he will merit salvation. If it is far from the time of Ge'ulah, he will merit Techiyas ha'Mesim, and merit Eretz Yisrael and good.
Malbim: You will be elevated over all physical matters.
Why does it say "v'Ha'achalticha
Radak: The matter is repeated in different words.
Why is it called "Nachalas Yakov Avicha"?
Rashi, from Shabbos 118a-b: Anyone who makes Shabbos a delight receives an inheritance without boundaries - "Az Tis'anag Al Hashem... v'Ha'achalticha Nachalas Yakov." It is not like the [limited] inheritance promised to Avraham, "Kum His'halech ba'Aretz l'Arkah [ul'Rachbah]" (Bereishis 13:17), or to Yitzchak "Ki Lecha ul'Zar'acha Eten Es Kol ha'Aratzos ha'El" (ibid., 26:3). Rather, it is like what was promised to Yakov -- "u'Faratzta Yamah va'Kedmah v'Tzafonah va'Negbah." (ibid., 28:14). Yakov guarded Shabbos - "va'Yichan Es Pnei ha'Ir" (ibid., 33:18) - he established Techumin (Bereishis Rabah 11:7). So explained R. Shimshon.
Radak: His children receive the inheritance [of Eretz Yisrael], and not Yishmael ben Avraham, and not Esav ben Yitzchak - "va'Ya'amidah l'Yakov l'Chok
Why does it say "Ki Pi Hashem Diber"?
Radak: Since He said, He will fulfill His word.
