I did that route about 10 years ago in two days. It was brilliant! About 75 miles each day and we stopped at the yha in Alston I think. Petty much slap bang in the middle (highest Market town in England apparently, but you get a lovely 5 mile decent to finish your first day).
My advice would be, don’t make the mistake that we did, which it doesn’t sound like you will, but anyway… we thought
It would be a good idea to use mountain bikes instead of road bikes so that we could do the alternative off-road sections. They were fun, but added way more time than we’d expected and didn’t get to Alston until about 22:30, and it was pretty dark! Made worse by the fact that we didn’t have any lights and I was wearing prescription sunglasses!
Anyway, have fun, ’twill be great!