That sounds like a great trip!
30+ years ago I walked from Lands End to Co Durham and averaged about 45lb on my back in a cheapo frame pack and covered an average of a little over 20m per day (shortest 6m, longest 30m). Naturally I was resupplying every few days but in those days everything was heavy (tinned food, heavy gear etc) - I was headed for JoG but had a fall (when not wearing the pack) and badly damaged my right knee so was forced to give up.
I hope you have a great time I loved all my various long walks
The PW crosses some great terrain