I just had my interview, and I think it went really well, lasted an hour and a half. I have 1430 SAT and 31 ACT, along with a laundry list of clubs and groups (one in which I went to nationals) but not a great GPA, so I really need any help I can get. How much will this interview help my chances?
Readers of your post don’t know the name of the school or how your academic and other parts of your profile. Consequently, responses would likely be speculation. However, a successful interview is good.
Congrats on a good interview 
Duke ‘considers’ the alum interview, but that’s all. In practice, as with most universities, the interview has more negative weight than positive (that is, a bad interview is more likely to affect the outcome than a good one).