Runner's Training Tips: weak knees

I have been having a slight problem with weak knees lately.  I have gradually been increasing my long distance runs (now up to 12 miles) and have noticed that my knees start to turn inward during the last few miles.  Sometimes they even bump together.  I know this is due to lack of muscle somewhere...but which group?  I was thinking that I need to strengthen my inner thighs and perhaps my hips.  I do strength training once or twice weekly as it is, and may just need to add some more variety.  Does anyone know of any specific exercises that would help me out?

Thanks so much!

I have always had my running shoes fit for me.  I am considered a neutral- no over or under pronation.  Atleast not in the past...

If you find your knees bumping into each other it is probably the Vastus Medialus Oblique which is the inner (medial) lobe of your quadriceps. At least that is the advice I was given by my physical therapist earlier this spring. I was told to do single leg leg presses, lunges onto a half ball (not sure if that is the right term, but it looks like half of a big excersize ball with a platform bottom) and bounds across a mini-tramp. Concentrate on form and not letting your knee collapse inward. It helps to do the lunges in front of a mirror so you can see when you start to get bad form.

Could be the shoes too. Especially if you have a lot of miles on them. Could also be your form when you run. Over the years I developed a gait where I ran with my feet in line with one another (think of walking a tightrope). This ended up changing me from a neutral to an over-pronator.

My gait is the same as rpetreccajr. In fact, sometimes my foot brushes against my other leg as I'm running. Gets annoying, but I haven't had any leg problems running like that.