For example, a 5:00 miler tries a 2:30 800 followed by 5:00 static rest, then repeats 4 times. I suspect this workout is much more efficient for developing threshold than a 20 minute tempo.
Or...
A 15:00 5k runner does 3x1600 in 4:30 each with 9 minutes static rest.
I really like this workout idea a lot, but haven't tried them.