The prepositional phrase in this sentence is "to the doctor". A prepositional phrase always has a preposition (in this case "to") and a noun, pronoun, gerund, or an entire clause. It can be longer than that, for example the noun can generate its own adjective (in this case, for example, "to the best doctor"). Prepositional phrases can usually be replaced with an adjective or an adverb. This one functions as an adverb, and answers the question "where" (where did I go).
