Hola Alice
I cannot find that specific sentence in the lesson you are referring to, but, if the translation in English was "Lots of people are walking in the park" it should say "están paseando" or "están paseándose". "Pasando" would only be an approximate translation but not as accurate as "walking/walking by".
If you can tell us where you found that specific question or example please let us know.
Gracias
Inma
