This is the 4123rd most frequent Polish word.
biegaczy
The Polish word "biegaczy" means "runners" (plural, genitive or accusative case).
In this sentence, 'biegaczy' is the plural genitive form of 'biegacz', and it is used to indicate the runners who participated in the marathon.
W maratonie wzięło udział wielu biegaczy.
Many runners participated in the marathon.
Here, 'biegaczami' is the plural instrumental form of 'biegacz', used to denote the runners the speaker conversed with.
Rozmawiałem z kilkoma biegaczami o ich technikach treningowych.
I talked with some runners about their training techniques.
In this context, 'biegaczy' is in the plural genitive case, employed to describe the runners being referred to as known by the speaker.