Excuses are just that, excuses.  I know just as well as the next, some days our bodies are tired.  You still need to get out and attempt though.  Who knows how you will feel once you start running, start pranayama breathing, or even just hit quickstart on the elliptical.  If you give it a try and then need to shut it down, you have evidence the body needs a break.  Until then, you just have an excuse.