Knee pain from what?

Hey all, Tri season is done for me this year. I have been training for the Grand Rapids Marathon lately and on my long runs I get knee pain behind my knee. I have been trying to figure out what is causing it. It happens when I am going over 14 miles the last few weeks. This doesn’t happen during Tri season only after when I train for Marathons in the fall. I am leaning towards unbalanced muscles as it is only behind my right knee. The muscles in my calves have been VERY tight and my hammys are always tight. I have been paying attention to my form and I don’t think that is it. During the fall I end up not riding nearing as much as during the Tri season which I know attributes to it.

I have also just purchased a foam roller which I used after my 20 mile run today. It is a good thing that my 5 month old can’t understand all the unique phrases that came out of my mouth when using it. Yeah, it hurt, but when I was done I could walk up the stairs without any pain. It seems like the 6 inches above my knee and 6 inches below it were REALLY tight.

Thoughts? Like I said, my opinion is possible imbalance in the hip area, and/or just calf & hammy tightness, but the prior is probably more the culprit.