probably not 'terrible' but 2A*s 7As 1B GCSE, then 4As at AS and 3A*s at A2. I don't think GCSE grades matter that much to be honest, a lot of people (like me) have a wake up call and realise we have to work hard at a-level. Ignore your GCSEs, you can't do anything to change them. Just tell yourself you're gonna get 4As, work like student who would get 4As, get rid of distractions (including friends) and enjoy yourself on results day, The hard work pays off, trust me.
p.s. you don't even have to be doing 18 hours a day from September, just never leave a lesson confused and always do your homework.