Hi Anji,
When I did the Camino last year, my first week was the most difficult because my body was still adjusting to the long walk and the weight of the backpack. I believe another reason was the number of kilometers I walked during the first week 27 kms the first day , 21 km the second day, etc. You're smart to plan to walk 15 km a day. Then you can pace yourself and walk leisurely. More than the kilometers, I believe you need to be more concerned whether there is an albergue or accomodation in the town you intend to spend the day after walking 15 kilometers.
Walking 15 kilometers a day would allow you to enjoy the moment of walking and not just rushing to reach Santiago de Compostela. There are just so many wonderful things along the way that you cannot afford to miss: the scenery and the people, the churches, the moments of silence or reflection or prayer, the fact of being alone and not lonely, the thrill of actually doing the Camino after so much hesitation, the pelegrinos you'll meet along the way and will soon become your friends, the food, etc... The Camino has lots of surprises, enjoy them! Buen camino!
Blessings of Peace,
Manny D.