So you confessed to doing 20 over. That was pretty stupid, and is going to be very hard to beat in court. Hope that he doesn't show.
Remember: Demerit points dont matter unless you're losing your licence. Fines for speeding are small. You're fighting to keep your insurance cheap. Admitting guilt to an officer is NEVER the right thing to do, especially when he admitted to you he had no idea how fast you were going. (Officers cannot "guess" your speed based on how fast they were going to catch you, that will never stand up in court.)