First of all I'm not a big fan of being a slave to the HR numbers as most people (unless you've had a really good Vo2max test done) don't even know their 100% max HR. Heart rate monitors are also not to be trusted at all times. People will run hard in a workout and think they hit 100% when in reality it might've been only 95-97%.
Of course during the course of a "traditional 20-min Tempo Run" you would probably look at a HR creep. Obviously the first 1-2min or so you will be pretty low, but then it will continue to climb (slowly) if you are holding the same pace/velocity.
So maybe you average 87%, but it started in the low 80s, hit 85-88% for most of the second mile, and then maybe spiked up to 90% or more near the end.
Traditional Daniels 20-min Tempo (and yes i read his first edition in middle school a long time ago), was that you want to hit the proper pace and hold it steady. It's a good workout during early base phase because it's not as hard as Vo2max intervals. It's also a good workout during the taper phase because it also is not too hard, but it's good to "maintain fitness." . Finally it is a good workout to do between some really hard sessions as it is more moderate and you get in some quality without as much risk for overtraining. You always want some element of Threshold (read Lactate Threshold) Training for most of the year.
True Vo2max intervals (and other much harder workouts that may be in the 5km-10km quality running realm) are used more sparingly to induce peak fitness.
A final note on the Fartleks (or even "timed intervals"). You can do a 30-60-min session (say 1-min on/1-min Easy) and have HR average be around "Uptempo" (read a notch easier than Tempo/Threshold), however on the 1-min fast surges you could be hitting a much faster pace (i.e. 10km pace even) and induce HR spikes above Threshold. So you have to look at the details to see the overall stimulus of the workout...and what the goal you are trying to accomplish.
In general though this is all very good "high end aerobic training." You keep things in control, you don't induce too much lactic acid at all and you improve your ability to utilize/clear lactate.
Of course when you want to peak and hit "race pace" at half marathon/10km and below or improve running economy you also need the balance of faster paced speed training and intervals (i.e. Vo2max "Intervals" and Daniels label of "Reps").