Five miles at 12:14 pace

My right knee is very tight and worrying me. Seems like it's always something... and I'm not even running that fast! First four miles at about 10 minute pace and the last mile I walked. Geez, doesn't this get any easier? Didn't help I was still full from dinner. Ran on the home treadmill in my new Nike Air Zoom Triax Structure 11s.