On a multiple day event, you've also got to factor sleep into your calculation. On a 1200K, I'll usually ride the first 400K, at least, all in one go. But it is in the remaining 800 km where I get my 5-8 hours of sleep.

Here are my average speeds on the Rocky Mountain 1200 ... the first 1200K I did. These average speeds include all breaks of course.
http://www.machka.net/rm1200/result.htm