Oohh, this is a little reassuring,(I haven't downloaded anything like that much) was there any time-frame? Could you specify anything as to that? Like the time of the offense to the time of you notifying?
If it makes you feel better, your ISP can't actually sue you. You're not infringing their rights. The worst they can do is cancel your service, and you'll just go somewhere else.
The ISP is just covering their rear so that if the RIAA catches you, they don't get caught in the lawsuit, so they're going around busting their customers and telling them not to pirate. Good for them.
As long as the RIAA doesn't find out that you are pirating, you are safe. Of course, tick off your ISP too much and they might report you to the RIAA, so I'd recommend listening to them