Are you doing your Tempo workout too hard, too easy, or just right? There's a simple way to find out, the two minute test.
As you progress in the distance or time that you run your tempo intervals, you will need to walk or jog between the intervals. Each week as you lengthen the distance from .5 mile to .75 mile to one mile lengths and beyond, the time that it takes you to recover should be the same.
Utta Pippig's coach followed the rule that the runner's heart rate should get down to below 60% of MHR before getting into the next internal or repeat. It's a good rule. But you need to also add to that equation the time it takes you to reach that level. Hence, my two minute rest rule.
It should not take you more than two minutes to get your heart rate below 60% of MHR. If it does, you are working too hard, either by running too fast, running too long [distance or time] or not slowing down enough between the intervals. If when doing the tempo runs [in the 80-89% of MHR range] you can't seem to reach approximately 60% of your MHR after two minutes of walking, change slow your speed or reduce your distance. A tempo run isn't supposed to be a speed workout. It's supposed to teach your body to maintain a pacing speed, not a quarter mile speed.
If you find that in your recovery time, your MHR drops from 80-89% of MHR to less than 60% in less than a minute, chance are you aren't running it fast enough or long enough. So, in this case, increase either to reach the more than one minute recovery time.
Either too fast or too slow is not the way to go. Like Little Red Riding Hood, you want a speed and distance that's just right. ;->