First off, excellent job stopping when you felt knee pain. I was always told to never run through pain stop and run another day. I am a runner who feels that if the log says 20 miles I would rather run 20.5 or more than run 19.5. I have to see the exact miles. (I am weird that way.) Since you are planning on running another 20 mile run, I would not count the 18 as a 20. I was in a similar situation a month ago. Running a 20 mile run and had to stop at 17 because of a leg cramp that got progressively worse. I chose to not run through the pain since it was only a training run. I did not count that as a 20. Similar to you, I still had another 20 mile, 22... just completed and a 23 in a week. I figured that lost 20 would not matter since I have pleanty more. Good luck on your next 20 mile run. Watch your form when running and please listen to your body.