The pins and needles when you sit down could be your piriformis muscle (this is in your backside) pinching your sciatic nerve. Try sitting on a tennis ball or golf ball and see if that offers any relief. There is also a seated stretch you could try and see if that relieves pins and needles. Take the foot of the effected leg (the one with the pins & needles) and place it on the opposite knee and lean forward and you should feel a stretch in your backside.
Knee problems and piriformis can be caused by overpronation - have you ever had your running gait analysed?
Try the above as Dale as suggested but it it doesn't get any better see a physio. You may want to consider some knee strengthening exercises as well. You may have a weakness in your quads. Have a look at this website
http://www.knee-pain-management.com/knee-exercises.html