When running track workouts, we use several identifiable
types and intensities of training :

1. Easy running (E) which includes warm-up, cool-down and recovery between faster bouts of work.  You also use E intensity on long (L) runs.  E running has a fair range of intensities – from about 60 to 75% of VO2max.  For just about everyone, E running is strictly running at a comfortable, conversational pace, which certainly will vary from day to day, depending on how you are feeling.

2. Threshold pace (T) is comfortably hard running for either a steady 3-4 miles (or 5 to 6km) or repeated runs of 5 to 15 minutes each, with 1 to 3 minutes of rest between the runs.   Intensity is to be 86-88%VO2max (a speed of running that you could maintain for about 1 hour in an all-out 1-hour race).    The idea of “Tempo” runs is to spend some time at T pace, so when a Tempo run lasts more than about 20 minutes, the earlier miles are to be a build up to the final 20 minutes, which should be at true T pace.  A person might even do a 10-mile “tempo” run, but this would mean starting relatively moderately and gradually speeding up until the final 3-4 miles are at T pace.  So, the longer the tempo run, the more time spent increasing speed up to that final 20 minutes at T pace. 

3. Interval-pace (I) running or “hard” running.  This is hard and at a pace that you could maintain for about 10-12 minutes in a serious race.  I work bouts are best if they involve runs of 3 to 5 minutes each (800 to 1600m are typical), with jog recoveries of similar duration (not necessarily, equal distance), relative to the runs they follow.  If a workout calls for “hard” runs, then go by feel and imagine 4k to 5k race pace (even 3k race pace for slower runners), as the intensity of each run.

4. Rep-pace (R) speed should be the same at sea level and at altitude, but with longer recoveries at altitude.  Recoveries are to be long enough that each run feels no more difficult than the previous run, because the purpose of Reps is to improve speed and economy and you can not get faster (nor more economical) if you are not running relaxed.  If it takes 3 minutes recovery between Rep 400s, then that is what is needed.  Reducing rest time between individual work bouts does not make for a better workout, in fact it probably makes for a worse workout because the short rests could increase the stress and lead to poor economy.  Think of R pace as equal to current mile race pace.

Strides are runs that last about 20seconds each and are run at a subjective feeling that approximates R-pace running.  Take 30-60 seconds rest between strides, or enough time to feel light and quick on each stride.  NOT sprinting.