This is the 4375th most frequent Russian word.
быстрым
"Быстрым" means "fast" or "quick" in English.
Here, "быстрым" is used as an adjective in the instrumental case, describing the noun "бегуном", indicating the quality (speed) of the runner.
Он оказался быстрым бегуном.
He turned out to be a fast runner.
"Быстрым" is used adverbially within the instrumental case, describing the manner of walking (quickly, with a quick step).
У неё быстрым шагом получилось прийти вовремя.
With a quick step, she managed to arrive on time.
In this instance, "быстрым" is an adjective in the instrumental case modifying "подходом", describing the approach as quick.