Runners are perfectionists. They want to make sure they follow their training schedule to a tee, run every tenth of a mile, never miss a workout, and especially not take a day off.
How do I know? Because I am a runner.
Please take this piece of advice:
Don't set your daily or even weekly training schedule in stone.
Many of us run each day according to a prearranged schedule. This is less than ideal because our body feels different on a day to day basis, not mention a thing called life can sometimes interfere and make it hard to follow our schedule.
Muscle soreness, illness, lack of sleep, commitments that must be attended to, and emergencies all come into play on a day to day basis.
If you are scheduled to do a workout but are still overly sore or only had three hours of sleep the night before, then your workout will be sub-par. Every workout should be done with a purpose and if you are feeling lathargic, extremely tired, and unmotivated to perform your workout, your body is trying to tell you something - you are not going to get much (if anything) out of completing your workout. Chances are, if you decide to do the workout, that not only will your workout be sub-par, but your next couple of days of training will be too.
Now I am not trying to say that if you don't 100 % then you should not run at all. What I am saying is do not be afraid to do 5 miles instead of your scheduled 8. Do not be afraid to to just run an easy 10 miles instead of your scheduled 10 mile tempo run at marathon pace. Do not be afraid to post-pone your track workout until the next day after you can catch up on some sleep to give it a true hard effort that will benefit you.
My point in saying all of this is to not set your daily training schedule in stone. Instead, have your plan but be flexible and be willing to make minor changes. Listening to your body will go a long way in not only allowing you to make your hard efforts count, but will help keep you injury free, and mentally fresh.