Learn Russian: Я пёк этот пирог всю ночь.

When you want to say how long an action lasted (its duration), Russian uses the bare accusative case, meaning no preposition is used. Since 'all night' tells us how long the baking went on, the dictionary form вся ночь changes to its accusative form, всю ночь.

Why do we use the imperfective verb пёк instead of a perfective verb here?

The phrase всю ночь emphasizes the continuous process of baking. In Russian, when you state how long an action was happening, you must use an imperfective verb. If you instead wanted to say 'I baked the pie IN one hour' (focusing on the final completion), you would use a perfective verb along with the preposition за.

I thought past tense verbs usually end in -л, like делал. Why is it just пёк?

The dictionary form of this verb is печь (to bake). Verbs ending in -чь have irregular past tense forms for the masculine 'he' or 'I' form, completely dropping the -л. So a male speaker says я пёк, but a female speaker would say я пекла (which keeps the -л).

Is этот пирог in the accusative case here, even though it looks exactly like the dictionary form?

Yes, it is in the accusative case because it is the direct object (the thing being baked). Since пирог (pie) is a masculine inanimate noun (a non-living thing), its accusative form stays identical to its nominative (dictionary) form.