Did you think that only one past verb form at a time was enough? Imperfect and preterite are often used together to talk about an action that was interrupted.
Yo disfrutaba un café cuando tú me llamaste.
I was enjoying a coffee when you called me.
You can use cuando to link the two actions.
Yo iba al mercado cuando vi a José.
I was going to the market when I saw Jose.