To ensure you get the most out of your time running, you should make sure you aren't performing a workout "just because". It pays to know why you're doing what you're doing.
Here's a problematic example: I want to run a half marathon, but I'm not sure how to train for it. The internet must have the answer, so I search and find 100's of programs I could use.
After careful consideration -- that is, picking randomly -- I decide to use a Hal Higdon program. Now I'm no novice, so I choose his intermediate plan.
This plan has a number of wonderful types of workouts. Everything from tempo runs, to intervals, to long runs. Rare for a plan, it also explains the types of runs which are to be completed. It explains the how of each run type, but does not touch on the why you should be performing it.
As I begin to follow the training plan, I run the pace it says I should run and hope for the best.
After the first month I hit a speed bump. I'm sick for a few days and skip the scheduled workouts.
Now that I'm off the beaten path