If it's to simply stay on the bike all day, enjoy the scenery but ultimately do little else then 75-80 miles a day
If however you want to meet the locals, enjoy a cafe and see what you are passing through then 45-50 miles a day.
I've led tours with both types of cyclists and I am definately in the 45-50 mile a day catagory.
The Lands End John O'Groats in the UK is a classic example.
It's a 1000 miles trip. For some a 3 week meandre is ideal. However for most it's a 10 day sprint and I personally think they are missing the point as if you talk to them they have nothing to say about the trip aside fatigue, but look on it more as an achievement.