If you feel tired, don't be afraid to stop and lay down somewhere safe and close you eyes for a 15/20 mins nap.
You should have plenty of time to do this so long as you keep your fuel stops short, and don't keep stopping to admire the view or take pictures.
If you are planning your own route, don't forget to get a receipt if you change direction. For example if you are riding up the M6 and decide to go into the peak district and then back to the M6 to add extra mileage get a receipt at your your corner point in the peak district.
These long rides are more of a mental challenge than a physical challenge.(pain is something just in your head
)
Good luck