A few miles into the town --- verbless clause, or adverbial phrase?
Solution 1:
Whether or not "being" is included in the sentence, it is an adverbial phrase. Including "being" or any present participle in the formation of an adverbial phrase that introduces a sentence simply makes it a type of adverbial phrase that's called an "absolute phrase." In this context, it is not a clause because "being" isn't functioning as a verb but as a gerund. The phrase, whether it has the gerund or not, adverbially modifies the verb "saw" in the main clause. It describes on how "I saw."