Workout intensities are based on how you breath and feel. Training plans include terminology based on Jason Koop’s Training Essentials for UltraRunning. Intensity levels (and a 1-10 scale) are as follows:
RecoveryRun (4-5): Comfortable breathing, just above walking breath. Should be able to talk all day at this level.
Endurance Run (5-6): Moderate-depth breathing that is rhythmic and not labored. Should be able to maintain a conversation at this level.
SteadyStateRun (7): Deep and labored breathing. Should be able to speak only 2-3 sentences at a time.
TempoRun (8-9): Deeper and more labored breathing. Should be able to speak only one short sentence at a time.
RunningIntervals (10): Short and rapid breath. Should be able to speak only one word at a shot.
Additional terms describe components of a workout:
Surge: a very short pick-up in intensity during a run
Fartlek: a longer ad-hoc pick-up in intensity during a run
Strides: a drill to improve your running form and speed (click here for an explanation on how to do running strides)