"We rarely run in the park."
Translation:Rzadko biegamy w parku.
Biec is just as an alternative infinitive of biegnąć, which means to be running at the moment in a specified direction (both conjugate in the 1st person plural as biegniemy). This sentence however is about a recurring event, so the verb biegać (1st person plural: biegamy) should be used. Biegać either refers to habitual action (as in this case) or can also refer to a real time event, when people are just running around without a specified (common) direction.