When time is short

I run an out and back just incase I have to retreat early. My knee has been sore lately and I was determined to figure out why. I bought some new shoes to see if that was the key to my discomfort but I had another idea as well. So I set out and found an easy pace to see what would happen. I've noticed that the pain seems to come at around 7 to 9 km into a run, so I didn't expect much early on. I pushed an early km hard to see how I felt and it was good. Almost like clockwork at 8k my knee started to twinge. I paid close attention to my stride and fatigue level. I counted my strides. I concentrated on footstrike and if I was pronating or something else. Just how was my foot hitting the ground, toes first or midfoot like I constantly practice? Seems that when I tire, my heelstrike becomes more pronounced and my stride becomes longer seemingly putting stress on my knee. So I attempted to shorten my stride and make sure that my foot hit the ground mid to soften the blow. In three km I had at least somewhat alleveated my knee pain. It was sore already but it wasn't getting worse and later on it did not hurt at all. So I will continue to concentrate on this as my running saga continues. 58:39 ~ 12.1 km, 4:52 per km.