It isn't "on the beach" because that is an adverbial phrase (it tells where you want to run), it isn't "the soft sand" because that is the object in the sentence, it isn't "have always wanted" because that is a verb (in present perfect tense) and it isn't "feel" because that is a verb as well.
Therefore, the prepositional phrase is "beneath my feet" because it is telling where the sand is in relation to your feet, which is what a preposition does.
