I did this walk last January, and found the worst part was the last couple of miles into Bath, because of the stste of the footpaths which were extremely muddy. This may be different in May. After you are past Bath racecourse, this problem eases, so don't get too disheartened at the start!
Most of the walk is a series of ascents and descents, and although one or two are worse than others, there's nothing too serious.This was one of the best walks I have ever done, and one that I intend to do again in the future. I'm sure you'll enjoy it.
I don't know of any south to north guides.