Maran, in Siman 306, discusses the prohibition derived from the Pasuk in the Navi: "Im Tashiv M’Shabbat Ragleha, Asot Hafaseha B’Yom Kodshi …". This Pasuk teaches that on Shabbat one is restricted from pursuing his own needs. We must engage only in matters pertaining to Hashem, Shabbat and the Misvot. The Hachamim derived from the word "Daber" (Talking) "Dibur Asur, Hirhur Mutar"- the problem is to talk about activities prohibited on Shabbat, but it is permitted to think about them. For example, one is permitted by the letter of the law to think about his business on Shabbat. However, this is not praiseworthy, since it detaches a person from the Shabbat spirit.
Another application of this Halacha involves walking to the city-limits on Shabbat for the purpose of performing a Melacha there immediately upon Mosa’eh Shabbat. Examples include wanting to set out on a journey, hire workers or receive an object which is Mukse. Although technically he did not speak about Melacha, nevertheless it is prohibited because of Mar’it Ayin-people will assume that he is walking to perform a Melacha on Shabbat itself. Moreover, walking for the purpose of Melacha is tantamount to speaking about Melacha.
However, it is permitted to approach the city limits for the purpose of a Misva, such as setting out on Mosa’eh Shabbat to perform a Brit Mila or tend to a wedding or funeral. Of course, one may not leave the Tehum Shabbat-the boundary of Shabbat.
Based on this, the Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908) and the Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) permit walking to a different street within the confines of the city for the purpose of doing a Melacha there immediately after Shabbat. Since it is not apparent that he is going for that purpose, nobody is likely to suspect his true purpose, as opposed to walking to the city limits. They will most probably assume that he is going to pray or to take a Shabbat walk. Of course, he is not allowed to verbalize his intentions.
SUMMARY
It is prohibited on Shabbat to speak about performing a prohibited activity, but it is permitted to think about it.
One may not walk to the city limits for the purpose of being in position to perform a Melacha immediately upon Mosa’eh Shabbat, but it is permitted to do so for the purpose of a Misva.
There is no problem to walk to a different street within the city for the purpose doing Melacha there on Mosa’eh Shabbat, as long as one does not verbalize his intent.
