Re:Knees begining to hurt
You could have a weak posterior chain and tight quads. Or you could have internally rotated femurs (do you stand or walk with turned out feet? - that can be a sign). There are quite a few things that could be a problem.
Check out restorativeexercise.com and katysays.com (her blog). She has tons of info on knees, hips, feet, etc. Her blogs have videos. I'm a real fan, as I've had success using her methods for PFD and also knee issues (I'm 64). Katy is a biomechanist and super smart.
Hope this helps,