Most athletes perform workouts at a single intensity.  They will warmup, do a single intensity effort, and then do a cool down.  If they are particularly inspired, they will do multiple reps at a single specific intensity (interval training.)  This approach, while convenient, can often leave gains on the table.  Running at multiple paces within a workout can have many benefits.  

First, it can teach your body to deal with how races play out.  It is rare for races to be raced at a single effort.  There are uphill segments that may require higher efforts or downhill segments which give a chance for active recovery.  The Athlete may need to cover a move of another racer if they have podium aspirations.  By training at different paces, we teach our bodies to handle the different paces that can occur during a single race.

Another advantage of running multiple paces in a single workout, depending on the paces chosen, is you can teach your body to utilize lactate as a fuel.  By working at paces faster than your lactate threshold and then changing paces around that threshold you can begin to teach your body to be more efficient at utilizing the lactate generated by the harder paces at the beginning.  

These workouts take getting used to if you are an athlete who is used to single pace, zone 2 and 3 training.  Tackle each workout in order and plan on at least one active recovery day following the workout.  Lactate shuttling workouts can be effective training for all race distances from the 1500 to the marathon if utilized within a well thought out training plan.

Three Workout Progression.

Important main set workout notes

1. ONLY take the given rest, no more
2. One pace flows into another.
3. Complete the workouts successfully before moving on to the next workout.  There is a progression from 1 to 3.
4. I refer to threshold pace (FTP) as the pace you can maintain for 1 hour.  Many people do a 30 minute test.  The pace should be about 90% of this pace.  If you do a shorter test you will need to modify accordingly.

Flex #1

Warm up
15 minutes last 3 up-tempo
Drills and Strides

Main Set
Repeat 4 times

1. 3:30 min at 106% of threshold (CV)
2. 2:30 min at threshold
3. 2 min recovery (very slow jog, try not to sit still)

Cool Down
10 minutes easy

Flex #2

Warm up
15 minutes last 3 up-tempo
Drills and Strides

Main Set
Repeat 4 times

1. 2 min at threshold (or around 400m)
2. 4 min at 106% threshold (around 800m) (CV)
3. 2 min at threshold (around 400m)
4. Rest for 2 minutes.

Cool Down
10 minutes easy

Flex #3

Warm up
15 minutes last 3 up-tempo
Drills and Strides

Main Set
Repeat 4 times

1. 3 min at 106% of threshold (CV)
2. 2 min at threshold
3. 2 min at tempo (~90% of threshold)

EACH SET FLOWS IMMEDIATELY INTO THE NEXT.  There is no rest, the 90% threshold is active recovery.

Cool Down
10 minutes easy

This is a hard workout!