What does "a 30-minute walk" mean in English for Spanish?

In English for Spanish, "a 30-minute walk" means:

30 minutos a pie

Listen to "a 30-minute walk": (If you have an HTML5 enabled browser, you can listen to the native audio below)

This is a phrase that is used in the GamesForLanguage English for Spanish Language Game in the following scenes:

• English for Spanish 4, Level 3, Scene 3