I was going to put this in the Spine Race thread but decided it was a bit off topic, though that was the thread that made me ask the question.
Watching the trackers on the website, it looked as if Byrness to Kirk Yetholm was a bit over 23 miles. In Pennine Way guide books it seems to be quoted as between 26 and 28 depending on whether you do the Cheviot detour.
As we have all been watching people running up the PW, and it is clear from the posts many of us have done the walk, I wondered if anyone could come up with the definitive answer. When I did the Pennine Way I split the Cheviot ridge and have always slightly regretted it. If I manage to get round to doing a North to South one day, I would want to do it in a oner. The difference between 23 and 28 miles is for me the difference between perfectly possible and unlikely!