Bonjour
She's a very fast runner
Word for word = C'est (Elle est) une coureuse très rapide.
It all depends on the context of this sentence. Are you talking about an athlete ?
If not I would say : Elle court vraiment très vite. (She runs really very fast) for example...
OR, Elle est vraiment très rapide. (She really is very fast)..
.....
Wish I knew the context of this sentence
☺☺☺