other uses as well, but the point is that nouns don’t exist in a vacuum. they have a job to do.
practice took a long time.
the word practice might look like an action, and in another sentence it might be a verb (i practice), but here we already have a main verb (took). so we can figure out that practice has to be the subject (the thing that took a long time), and only a noun or pronoun can be the subject. 