Running's Dirty Little Word

There is a word or phrase in running that is dirty- no runner wants to say it and no runner wants it associated with them.  DNF!  Did Not Finish!  It happens to lots of runners, even the elite have DNF'd.  While I am now associated with that saying.  I attempted to run the Laugh and a Half Marathon in Norfolk this past weekend.  I felt fine that morning no problems to be concerned about it.  My main worry was the temperature and even that turned out to work in my favor as it was a great morning to run.  The first half was just fine.  I put my watch in my pouch and just ran how I felt like running.  I even chatted for about a mile with an elderly gentleman from Clarks.  He has ran a marathon in every continent, every state in the USA and is working his way through Canada.  You could tell he was just out to enjoy the morning. 
When I came around the half way point the clock said 51 minutes.  Wow!  I am booking along.  I felt great, waved at my sister in law and just kept going.  While about mile 7 my knee started to hurt so I stopped and walked a little bit.  When I started to run again it started to hurt again.  I thought I could run through it and loosen it up but it just kept getting worse the more I ran on it.  I spent the next 2 miles or so wondering do I DNF or do I just keep going?  Since the race was a relay for some runners there was a trolley at mile 10 to take the runners back.  I decided in the end this one race is not worth me ruining some of the other events coming up this year so I DNF'd.  Lucky for me there is no record that I DNF'd.  I just never show up on any results.
I am not sure what happened.  The road surface wasn't the greatest so maybe I just twisted it.  I have really been running a lot this year so maybe my body is telling me to take some time off.  I am not sure if I will ever know why it started.  It is still sore and stiff a couple days later but I am hoping it will just get better and I can go back to my happy, merry running self.  The pictures below are about where I started to be in pain.