I started in Irun and walked to Burgos.
But the first part very "off the beaten track" (because I chose a very alternative way through the Basque mountains).
Parts of the way were common: between Irun and Hernani, Zegama and La Puebla de Arganzon.
I liked it a lot, but it was VERY lonely...
As to the part "la puebla" till "Burgos", it was quite weird, but I keep a beautiful memory of it - "weird" partly because I felt "very abandoned", partly because some stretches were along awful national roads where the trafic was simply frightening. (now, with plenty of distance, I think it was a minor part, but at the moment of walking, I felt very bad.)
This might be not very clear but in my head it is/was not very clear...