I run 5-6 miles everyday?!

I run 5-6 miles everyday?

I am 42 years, 5'8" and 78 Kg. I have been running regularly for the last 20 years. I participate in 3-4 half marathon events every year and complete the distance in less then 2 hrs.
I run on various surfaces ranging from grassy tracks to tarred roads. I wanna know if that much of running is going to adversly effect my joints. I feel a bit of pain in the knee joints after running for more then 8 miles thats why I use knee supports whenever I have to participate in Marathons.

Hellloooooo knee replacements. You're probably tearing the crap out of your cartilage and you're probably already grinding bone on bone.