'Fast' in this example is an adverb as well . Quickly and fast are both adverbs here. The are almost synonyms . Both describe his running ( describing the verb 'to run' in this example , therefore adverbs) as being speedy, rapid and swift .

Fast is an adjective while quickly is an adverb.. also fast refers to speed while quickly refers to time.. for example: That car is very fast The car quickly pulled over :)