An average 12 car is usually 60 miles in 2 hours.
So your average is 30mph, however that includes that time sat at control (you can arrive early if it's not a regularity, if it is you have to keep to bang on 30, no more no less) and does not account for you getting lost. Which if you have coded routecards you will do. Making up time is the key.
The worse you are the faster you have to end up going. 12 car is all about the navigator, it's bloody good fun though. After getting lost and making up time along whites at 80ish a sensible speed is fantastic.
Night rallies are slighty different, they are more driver oriented, they have sections that are fully competeritve (so a couple of laps round a route on an airfield), competition sections along roads.