Why do my knees hurt after running? How to prevent it? 10 points!!!?

i run with my brothers every night. i usually only run a mile maybe a mile and a half. well yesterday, i really felt like running so when i got to the track, i had just planned to run a mile. i did ( a consecutive 4-lap mile) then i walked 2 laps ( 1/2 mile) then i felt like running another mile and before long, i decided to do four more miles. so i basically ran a consecutive mile, walk 1/2 mile, and ran 5 more consecutive miles. (6 altogether) well i drank about 20 grams worth of a protein shake and went to bed. when i woke up my back hurt a little bit but it feels fine now. but my knees hurt and i just want to prevent this...