Upvote:3
While I am not a lawyer of any kind, I don't think what you did is likely to attract much penalty, although the theoretical maximum penalty is quite high. While it is technically lying on an immigration form, you didn't actually gain any benefit from it. The the thing you achieved, entry to the US, was something you were entitled to anyway.
If this is now all in the past, I would let sleeping dogs lie. Never use the ESTA again. If you can cancel it without explaining why that might be a good idea.