This is a 9miles moderate to hard walk over paths, roads, mud, grave stiles, and steep hillsides. I would allow 7-8 hrs for this walk. This is real walk good footwear and ensure you have all the equipment you may need with you when on the site of the mountain things can change quickly there is a number of shops you will pass worth checking your water levels.
There load of good history and information in a printout form that can be found at:- https://www.torfaen.gov.uk/en/Related-Documents/Countryside/Torfaen-Walks/In-the-Footsteps-of-Pilgrims.pdf