The difference between simple past and past continuous is that the former is used to refer to an action that happened sometime in the past and was of a short duration, while past continuous is used to refer to an action that was in progress in the past:

Ο αδερφός μου περπατούσε κάθε πρωί στο δάσος.
Μy brother was walking every morning in the forest.

O αδερφός μου περπάτησε χτες στο δάσος.
My brother walked yesterday in the forest.

Simple Past may be used in combination with Past continuous to denote an action that ended in the past, while the other was still in progress:

Όταν η Άννα γύρισε στο σπίτι, ο Γιώργος μαγείρευε.
When Anna got home, Giorgos was cooking.

Όταν φτάσαμε στο σπίτι, η Ντίνα ετοίμαζε την βαλίτσα της.
When we got home, Dina was preparing her suitcase.