Hashem appears to Moshe in our parsha and tells him that he is going to redeem the Jewish people from Egypt and take them to Eretz Yisrael, a land of milk and honey (3:8) Hashem then tells Moshe to tell the people that the sign/proof that is the fact that Bnei Yisrael will come and worship Hashem on the very mountain that Hashem appeared to Moshe on (3:12).
R' Moshe Tzuriel in his Derishas Tzion asks two simple questions: 1) If Moshe is giving a sign to establish his bona fides and get everyone onboard with Hashem's plan, shouldn't it precede the events of yetzi'as Mitzrayim? What good is the sign of "ta'avdun es haElokim al ha'har ha'zeh" if it happens only after the exodus is complete? 2) How can "ta'avdun... al ha'har," kabbalas haTorah, all 613 mitzvos, be just a sign, while the one mitzvah of settling Eretz Yisrael be highlighted as the goal of the entire redemption?
R' Ya'akov Emden asks a similar question with respect to the nusach of birchas ha'mazon. We begin "nodeh lecha" by thanking Hashem for "eretz tovah u'rechavah," for Eretz Yisrael, and only later in the bracha give thanks for "al torascha she'limadtanu," for Torah. We received the Torah 40 years before we got to Eretz Yisrael -- shouldn't our thanks for it come first? Isn't Torah the primary thing we should be giving thanks for?
R' Ya'akov Emden answers that coming to Eretz Yisrael may be just one mitzvah and may chronologically have happened after matan Torah, but in terms of tachlis, in terms of defining our purpose and goal, it takes precedence over all else. If the purpose of Judaism was just to learn Torah and do miztvos, we could have gotten on very well in Mitzrayim doing 612 mitzvos and learning Torah once the burden of slavery was removed. But that's not what Hashem wants from us. He wants us to be a nation, which means living in our own country under our own sovereignty. That's the goal. Ramban (VaYikra 18:25) writes that the ikar kiyum of all mitzvos is only in Eretz Yisrael. What we do in chutz la'aretz is just to keep us practice until we can return to the ideal. Therefore, we first give thanks for Eretz Yisrael, and then go into the details of everything else.
When Moshe told Bnei Yisrael the sign of "ta'avdun es haElokim al ha'har ha'zeh," it was not designed to prove to them that they would leave Egypt -- it happened too late for that. It was a sign meant to inspire them to the real end goal -- to come to Eretz Yisrael.