The first sentence is OK because "fast" is an adverb. And the sentence example you wrote is incorrect, "We are early. I didn't expect us to get here so quick." because the adverb "quickly" should be used instead of the adjective "quick." Examples of quick in a Sentence Adjective They had a quick drink at the bar. She took a quick shower.

We are early. I didn't expect us to get here this fast. We are early. I didn't expect us to get here so quickly.