As Owling says, around 80 miles or so. A lot of Riverside walking, and I presume some harder stretches, and some genuinely stunning sections. Plenty of great places to stay, Ilkley is a nice start, Burnsall, Grassington, Kettlewell, Dent, Sedburgh, as well as some hidden gems - I drive along it and love it, presume a fair bit depends on the weather - went through a fair range today. Some great pubs and B&Bs to stay at.
Apparently you have to dip your toe into Windermere to say you've done it.