"Fast" is an adverb?!

For years and years and years, I have gone by the rule that "fast" is an adjective and "quickly" is an adverb, maybe with a few exceptions if you're using "fast" in some technical sense. So "The horse is fast," but "The horse runs quickly."

My research involves adverbs. And several grammar sites are telling me that "The horse runs fast" is perfectly OK.

Wow. I mean... wow. Years and years and years.

"Good" and "well"... those are still distinct, right? Right?