Purpose drives the run
The first question to ask yourself is, "What is the purpose of this workout?" With a vision of what needs to be accomplished, you can rid yourself of guilt and indecisiveness, staying on the right path to your goals.
Perhaps you feel guilty that you only have thirty minutes for your easy run today. Is your guilt justified? Well if the purpose of the run was to gain weekly miles and increase your overall fitness then you may need to accept that you only can run for 30 minutes today and will need to add on the extra missing 20 minutes to another easy run later in the week. However, if the purpose of your run was to increase blood flow and recover from a hard workout the previous day then 30 minutes of easy running will fulfill that purpose very well. The extra 20 minutes that you will miss is unneeded running and be grateful that your busyness is keeping you from overtraining.
Or say you are returning back to training and your workout calls for 4 sets of intervals. You complete three of the sets and feel spent. You could eek out the last one using your mental superpowers. However being satisfied with the three intervals you completed will cause improvement without taxing your body to the point of overtraining or injury. This is when you become the smart runner who performs a solid workout and leaves the last one for another day. Your purpose was to work hard at your fast pace. Mission accomplished.
A favorite for runners is "The day off, I earned it" purpose. That is the day where you either read an entire book, wander through your home taking the time to do as you please, or snatch the coveted nap in the sunlight coming through the window. The purpose in this day is pleasure and recovery. It keeps you from regretting all the devotion and time you put into running.
The important question you must ask yourself each day is, "What is the purpose of this opportunity?"
It takes guts and wisdom to run with purpose.