Hello, I want to participate in a 322km ride on June 22. It's non-competitive, however I only have 20 hours to complete it.

I've ridden a 180km ride before (23.5km/h average ) and 170km (25.5km/h average)... however this is a new year and I want to try the 322km.

I'm planning on doing a 200km before to see if I'm ready, any other advice?
PS I'm 16 so I'm in school and it's hard to do too many 100km+ rides