Quickly and fast

[Ju]

I found the following sentence in a grammar book.

"Let's walk ( ) to the bus stop.


My answer is fast but the model answer is quickly.


May I know the reason?


Thanks.

 

[MikeNewYork]

One needs an adverb there and both words are adverbs. The only difference would be personal preference and style. I would personally use "quickly", but that doesn't make "fast" wrong.