You also need to factor in the fact that on much of the route from Paris you will be following the GR routes, which are anything but straightforward. They wind around a lot. Of course, you can make up your own route, too, but accommodations/food can be a problem.
Also, you don't have much of any choice in great swathes of the route in France about how long your stages are, because there is just no place to stay in many of the little villages you pass through, and often no shopping, either. My edition of the LePere guide lists 42 days from Paris to SJPdP, and he's totally cool with averaging 34 km or more a day in places like Les Landes.
If you are from North America, you need to take the Schengen limits on how long you can stay in the Schengen group countries into account. I could not have comfortably completed the total distance in 90 days, and had planned to split it into two years. Had I not broken my foot I hoped to get as far as Roncesvalles the first year.