I can't speak definitively to your suggestion about "drunken" (though it sounds acceptible to me as a native English speaker).
For your question, you are almost correct. The preposition ad ("to", "toward(s)") takes to accusative case so "to the forum" is ad forum (not foro).
However, the second phrase is correct. The preposition a(b) ("(away) from"; the word depends on the initial sound of the subsequent word, similar to the usage of "a" vs. "an" in English) takes the ablative case so "from the forum" is a foro, which is what you wrote.
Note that if we wanted to say "from the city", we would use ab instead of simply a), i.e., a foro and ab urbe.