The last time I did it took me 19 days. I found it easy to find campsites, shops, pubs and wild camping places as well. The weather can be changeable, I've experienced both good and bad. I have also hiked it both ways, my personal preference is North to South, for two reasons, my son lives in Edinburgh and when finishing at Edale I can jump straight on a train to go south to where I live in Portsmouth. I bought the A to Z books to hike it, really good for the route, A5 sized so really easy to carry.
I remember certain sections which required a bit more from me, Kirk Yetholme to Byrness, Alston to Dufton and Cowden to Edale for instance, the wind and rain really knocked me about on those sections. The Hadrians Wall section I thought was a bit strenuous as well, lots of ups and downs but great if lucky with the weather.
I realised this year that if I ever want to do it and MORE (which I must admit for me now is most unlikely) and make it into a REAL he-man's effort and adding about 9 days to the route, I would start to hike at Edinburgh. Follow the route to Kirk Yetholme, via Peebles and Melrose by using the route laid out in
www.walkhighlands.co.uk.
The other option which I've done. almost as long, is the Pennine Journey.