adverb
briskly; at a quick pace
Describes walking or moving quickly, often with a sense of purpose or urgency.
彼は足早に駅へ向かった。
He headed briskly to the station.
足早に歩いて、約束の時間に間に合った。
I walked at a quick pace and made it in time for the appointment.
速足 is a noun meaning 'quick steps' or 'fast walking', while 足早に is an adverb describing the manner of walking briskly.
Compound of 足 (foot) and 早 (fast), with the adverbial particle に. The exact historical derivation is uncertain, but the meaning is transparent from its components.