I have a book written in I think the 50s (at home, I'm at work) which describes walking the ridgeway. Apparently the largest material difficulty is water. As in you have to come off the ridgeway, in some cases several miles off the ridgeway, to find an available source of water.
It's hard to imagine that our great ancestors would not have provided water along the way in some form if the Ridgeway was always intended to be a long distance route just for traders and the movement of armies as has been suggested. Unless of course it was originally intended to just link up local communities and not as a major neolithic highway but just grew in size. In that case water would not have been a prerequisite as the then farming community (supposedly) would have no reason to travel huge distances.
I have two books on the Ridgeway Path, The Oldest Road and Elizabeth Cull's Walks Along The Ridgway and also R.Hippisley-Cox's 1914 book The Green Roads of England that gives it great mention and is a joy to read.
Yes, "The Green Roads of England" is a little gem of a book and very much evokes a different time. I walked up to the Ridgeway yesterday from Bishopstone which just below the spring line, on the way back we actually found the spring which feeds the streams and village pond in Bishopstone. I believe there are springs all along that stretch of downland - I know there is Woolstone spring at the bottom of Uffington Hill. Ancient travellers must have come off the path as juamei said .. it also begs the question of how did people get water to hillforts as many of them are close to the Ridgeway.