I've done up to 24 miles in one day on a day hike, and when doing long distance paths around 20 miles a day for 3 days, and that frankly was enough for me. I generally prefer an average of around 18 miles a day where possible, 20 if necessary based on campsites (I'm not confident enough to wild camp) and time available.
In general though I would say you need to work out a few things in advance, namely how far you can both realistically walk in one day, and why you are doing the route. If you are simply going to walk the route for the challenge of finishing it as quickly as possible, then you'll need to get in plenty of practice in advance. If you actually want to enjoy the route, take photos, see any nearby sites and so on then you should relax and take 5-7 days to complete it and not worry much about speed.
I generally prefer to take my time a little, that way I enjoy the walk more and have plenty of photos to remind me of what I've seen, but each to their own.
If you can, try to get hold of the National Trail Guide for the route because they're a great guidbook with lots of information and O/S maps included. I used the NTG when I did the Hadrian's Wall Path and only took a wrong turn once. Sometimes you can pick up older versions of the book in discount bookshops for £5. Whilst nothing can really replace full O/S maps, if you're not looking to make any diversions from the route the book will be all you'll need.
Edited: 30/10/2011 at 19:00