Dear Nuritan,
"We walked a couple of streets".
"A couple of" is "two". For example, "A couple of apples" = "2 apples".
In my opinion, this sentence is the same as:
"We walked the length of a couple of streets". (two streets)
"For exercise, we walked up and down two streets".
"We walked a short distance".
However, in informal use, "a couple of" means "An Indefinite Small Number", (Oxford Dictionary of English).
Therefore, as my third sentence shows, please note that, "a couple of" sometimes means "a few", "not very many", as:
"He �ll probably be in bed sick, a couple of days". ("He �ll possibly be in bed sick 1 day, or 2 days, or maybe 3 days, but for only a few days".)
I hope that I have helped.
Les