"Lauf du vor, ich komme hinterher,"?
Why is there 'du' between 'Lauf' and 'vor'? Is this an imperative sentence?
Yes, it's imperative.
The du is to emphasise that the command is addressed to du (and not anyone else); here, specifically, to set up a contrast between what du is supposed to do and what ich is going to do.
In English, it would be, "You run ahead; I'll follow behind" with explicit "you" before the imperative.