Look, I realize it's a bit informal but "She goes/walks quick." is just as commonly used as "She goes/walks quickly." Usually only in the context of directly remarking on something in the present form. One would not, I admit, use this in the past tense but we're quite clearly working in the present here.
Sie is indeed the formal form of "you" in German (taught in a later lesson), however it conjugates the verb differently:
sie geht = "she walks"
sie gehen = "they walk"
Sie gehen = "you walk"
So while there are some situations where it's not clear whether sie or Sie is meant (due to it being capitalised at the start of a sentence, or spoken), whenever you see the verb conjugated in the er/sie/es form (usually ending with -t), it must be sie as in "she".