About 15 years ago I did that (except it was a ferry), and as a 'hook' to pin the route on, I just wandered around the Commonwealth War Graves between Calais and Paris.
(google 'Commonwealth War Graves Commission' for all the info you'll need about where they are)
Without exception, as far as I found, they are all in areas of unspoilt country scenery, well served by small villages and B&Bs. (Although I camped most of the time). Pretty well every picuresque town has a municipal campsite.
I don't remember how much time I took - 2 or 3 weeks, but pedalled much less than 50 miles per day.
A clockwise route roughly around Arras and Compiegne, to Beauvais, returning around Abbeville and Boulogne - well away from the N roads. I think Compiegne was the only big town I went through, and was worth it (nice place).
Can't say much about preparation - just took my 'everyday' bike, puncture kit (couple of punctures I think) and rear panniers. And a fairly detailed map of that area.
I must do it again sometime!