"I don't like getting up early."
Translation:Nem szeretek korán kelni.
Is "felkelni" a valid infinitive as well? If so, is "kelni" a shortened version of it?
"Felkelni" is the valid infinitive form of the verb meaning to get up. "Korán kelni" is a phrase which means to get up early, even without the "fel" preverb. With other preverbs, the verb "kelni" gets other meanings.