Weak knee/runners knee?

I'm a very avid runner to say the least....

Ever since I started 6 years ago, I've ran everywhere I've went - never could stand walking; spring, summer, fall, winter - didn't matter what season or weather, I've ran through it all. I was avg. atleast 30-50 miles per week at my peak - typically ending weekends with half a marathon run.

Anyways up until 4 or so months ago I decided to do two back to back 15 mile runs - ever since then I've had a strange 'injury' of sorts on my right knee. My knee felt weak and a minor amount of pain - I looked up into it and figured it was runners knee so I took a good month off of hardcore running (severely aggravating for someone like me) and sure enough my knee felt up to speed again and I've taken off with a big smile on my face;

However there's always this sensation that my knee feels somehow "weaker" than it use to be - I cant quite describe it. If it was 'pain' then it is the most miniscule if even noticeable pain I've ever had - the knee itself just feels off. Whenever I extend it while at rest, the knee joint makes almost a 'grinding' underneath the skin. Even though its so benign it still had gotten me rather nerve-wrecked because of future plans on being in the military, I need the integrity of my knees to be flawless.

Because of it I've been taking the time to fully warm up prior to running - even started to do a daily 'yoga' in the morning the fully get my body prepared for the day; when fully warmed up, my running is flawless, I feel no discomfort of weakness - but as soon as I stop that weak sensation returns again. Because of this I've dropped the mileage a week to about 15-20 and been doing so less on concrete (I've noticed no decrease in power or speed)

What exactly is this? Am I going to have problems in the future or what's the deal?

I've heard the controversial "running on asphalt" - and "running ruins cartilage/knees" stories a lot but I'd like some straight up answers from anyone with REAL running experience.