Should I Train By Time (Duration) Or Distance?

Your planned workouts use time (duration). So please follow the workout guidelines relating to duration rather than distance.  

Why are there distance guidelines then?

Training Peaks automatically creates distance guidelines for your planned run workouts, based on the duration of the planned workout and your Threshold Run Pace in your Settings. Therefore, the planned pace and duration are only useful if your Threshold Settings are accurate. 

For the sake of consistency and reliability it is better you train by duration (seconds, minutes, hours) rather than distance. 

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