Replying to Quick Question About Running.

So, I started running at the beginning of the summer, 2 miles a day. Gradually I upped from 2 miles to 3, 3.5, 4, and now I'm running 6 miles everyday and have been for the past 5 weeks. I'm in the best shape I've ever been in and it really helps get rid of my college stress.
However...
Some of my friends are telling me that over 4 miles a day everyday is overkill...even though I feel absolutely great. So, NS...is it overkill? What are your opinions on running 6 miles a day everyday? Should I worry about future health issues in my knees/joints/etc?