There are several threads here on altitude. Use the Search function to find them. Thinking that 3 days at 11K is going to acclimatize you for 15K is unrealistic, in my experience, which includes living at 11.5-13.5ft in Bolivia and going to Macchu Pichu.

You may not be affected, but on the other hand you might. If you are, take it seriously. I was hospitalized twice with what may have been HAPE (high altitude pulmonary edema). It has similar symptoms as pneumonia, so I'm not sure which it was, but both times required what I would describe as intensive care - oxygen and drugs.

Being in good shape or not, surprisingly enough, may not determine whether or not you are affected.