Don and Dulce,
In the case that you have not gasped the theme here, Miles per Day is:
What you have to do because of the situation....
What you can do comfortably....
Our antique coach is very comfortable to drive, even still, we find 8~9 hours is plenty in any one day. In times past, when we had to do more, we did more. We also frequently co-drive. Having a real navigator in the right seat takes a lot of the load off the driver.
The important thing to always remember is that driving tired is an easy way to get dead.
Matt