Extending your stay

hi! I was issued a Schengen visa by the Spanish Consulate in Manila, Philippines. I intend to walk the Camino Frances in stages and this Sept, fr SJPdP to Burgos so I was allowed to stay for 21 days as stipulated in my visa. My visa is valid from Sept 1-Oct 5. What if I decided to continue my Camino from Burgos? What Spanish agency should I go into while I'm still in Spain to request for an extension of my stay. This may be a hypothetical question for me but not to others. Thanks.