If you think you might want to obtain a Compostela, them I recommend starting at Leon. On the other hand, if you want to walk from the beginning, with average fitness level, and accomplish the entire
Camino Frances say, in two trips, then starting from Pamplona is a excellent idea.
My wife made me promise no more walking up and over the Pyrenees after my first two Caminos from Saint Jean Pied de Port. That first day is a bit steep, for 8 - 10 km. Then it levels out and heads down and into Roncesvalles. So, the next time I consider doing the
Camino Frances, it will likely be with a start at Pamplona. There is excellent train service, and multiple flights daily from Madrid.
You should be able to make it from Pamplona to as far as Carrion de Los Condes in 13 days (+/-). Leon would take about 17 days (+/-) from Pamplona. You can get bus service from Carrion to someplace with rail connections to the rest of Spain.
For reference, Santiago is about 13 days (+/-) walk from Leon. Doing the second half into Santiago would qualify you for a Compostela. Walking from Pamplona to someplace short of Santiago does not.
I hope this helps.