A Runner's Guilt

Today I am feeling guilty because yesterday I skipped a planned run.  At the time I had a good enough reason for not running.  It wasn't that I was too busy or doing other training instead.  No, I skipped my run because my legs just felt too knackered.  My scarred right knee wasn't feeling too great, so I suspect my fall last week hurt it more than I realised.  I had also spent Monday doing loads of kettlebell lunge and presses, which look like this:

It's a reasonably demanding exercise, working the legs, core and upper body.  I was practising the exercise at home Monday morning, and then teaching it in three classes that day.  So it's hardly surprising that my legs were a little tired.  At the time I was convinced that having a day off was the best thing to do.  So why do I feel guilty now for missing a run?  Does anyone else get like this?  I'm even telling myself that I will make up for it by running on Friday instead, which is usually my rest day.  

My right knee still isn't feeling great.  It feels really stiff at the start of a run, but seems to loosen up after 5 to 10 minutes.  It will have a test later on today when I take the ladies from the Sweaty Betty running club for a training session.