What was Moshe referring to when he mentioned "Eleven days from Chorev"?
Rashi and Rashbam: He was pointing to the damage that they had caused by sending the Meraglim, 1 as a result of which they had to travel round Har Se'ir for forty years, 2 particularly bearing in mind that Hashem was so keen to take them into Eretz Cana'an, that he curtailed the eleven days journey 3 from Har Chorev to Kadesh Barne'a, to three days. 4
Ramban and Seforno (on Pasuk 19): The Torah is telling us that Hashem took Yisrael through the great and fearful desert because it was the shortest route from Chorev (Har Sinai) to Kadesh Barne'a (the outskirts of Har ha'Emori) 5 via Har Se'ir, and that they then wandered in the desert for forty years (due to their sins).
Hadar Zekenim, Rosh: Had we gone straight to Eretz Yisrael, it is only an 11 day journey. We delayed it, and surrounded Har Se'ir many days.
See Sifsei Chachamim DH 'vE'Ein Lehakshos.'
Seforno: Wandering aimlessly from place to place. In fact, Moshe pointed this out each time they encamped in one of those places - instead of going straight to Eretz Yisrael.
Seforno: Which He did by leading them through the desert, instead of taking them along the regular route via the land of Edom.
Rashi: From the 20th of Iyar when they left Har Chorev, till the 29th of Sivan, when they sent the spies - minus the thirty days that they spent in Kivros ha'Ta'avah and the seven days that they waited for Miriam in Chatzeros (See also Rashbam, who disagrees with Rashi regarding some of the details).
Ramban: Which in turn, belonged to Yisrael.
Here it says that it is an 11 day journey from Chorev until Kadesh Barne'a (Midbar Paran, from where they sent Meraglim). In Shabbos (89a-b), it says that these are the same!
Moshav Zekenim (Bamidbar 13:21): Chorev and Kadesh Barne'a are both in Midbar Sinai. It is one big Midbar. Different parts of it have different names.