Why doesn't tempo pace improve my fitness?

Hopefully in the last 17 minutes you've googled this, and found that there are many books, blogs, articles, entire websites dedicated to talking about this.

If you're serious about it, read some information published by people with names, don't rely on random nameless strangers on LRC

Variety is the spice of life.

Tempo runs do improve your fitness, but not if that's the only thing you're doing. The oversimplified version is that you have all these muscle fibers just waiting for their big day but you never recruit them because your body is dialed into your routine. How to find them? Run faster or longer than you're used to doing. That's what most training plans are doing for you. I think the two Petes (Pfitzinger or Magill) are probably the best, most engaging writers to explain this in more detail. Check out their books.

6:20-6:40 is too slow for a tempo run. Your tempo pace should be about 30 seconds/mile slower than your 5k pace, and you do 20-30 minutes at that pace. The idea behind this is that it improves your body's lactate buffering and in turn improves lactate threshold which makes it easier to run faster without getting as tired. 6:20-6:40 can be your pace at the end of a fast long run, but there's no reason to do other runs at that pace when training for 5k and 10k.

ya brah get off YouTube and hit the books brah.

6:20-6:40 is probably closer to a steady state tempo pace for you which has minimal gains. For sustained longer efforts at this pace you might gain threshold stimulus, but it’s almost a waste if you’re not preparing for a marathon and not doing any other real threshold-adjacent work.

Your threshold pace is probably closer to 6:00 flat. You could do intervals on the faster side 5:55-6:00 or longer tempos starting at 6:15 and getting down to 6:00 by the end.

Tempos aren't specific workouts for 5k/10k

If you're focused on 5k/10k, you should do at least two interval sessions a week, and drop the tempos and long runs outside of the base phase

A 36-flat 10k would project to 6:20 pace for a marathon. 6:20-6:40 pace is on the slower end of the spectrum for tempo work for that kind of a runner. 6-flat is closer to threshold. Tempo work more like 6:00-6:20 range.

If that seems aggressive, build up to it. Do shorter reps with short recoveries -- 3:00 reps/1:00 recoveries. Do broken tempos so you can hold a stronger pace. 3 x 10:00 w./ 2:00-3:00 ez jogging instead of a straight tempo. Extend the time you're spending in your tempo zone. If you're just doing a 20:00 session, build it up to 30:00-40:00. Stuff like that.

try to avoid use of the term polarized.

the reason the 6:20s aren't helping is you lack specific strength conditioning to push quicker paces; fix,you need to start a 6:20 mile rep every 8' (walking rest) then replace a few jogging workouts each week w 4-5 mile x 6:20 w 1:40 walk rest, basically adding overal ltraining work in the legs, but also impetus for respiratory muscles neeed work is also lacking. A big part of this is workouts adjustment I suggest is O2 kinetic work which you are short on, such work requires initiatingh reps not necessarily continuous medium effort work.

Because you are like me :)

I need to run once per week 3-8k pace intervall workout or do a race, otherwise I get slower.

Try 8-12x400 , 6x800, 5xk, 4x2k with a lap jog in between or 6x90 sec Uphill

Keep the tempo run in your training it is beneficial.

Good luck

Whats the difference between tempo and threshold?

"Tempo" and "Threshold" are used interchangeably across many coaches/programs. Some coaches call marathon pace runs tempo runs whilst others say threshold is a tempo run. The key difference is the duration at which you do these workouts and the stimulus you're getting. Marathon pace work is what it sounds like, running a workout at your predicted marathon pace. It gives an aerobic stimulus increased from something like steady state or easy running, but you can do it longer in sessions compared to threshold work, which is typically something you can hold for an hour. Continuous threshold runs can be 15-30 minutes in an adeuqate session or be broken up into timed/distance intervals with short rest. Marathon pace is almost always continuous and when applied to a training program, often takes up about 10% of weekly mileage. Threshold should probably be closer to 5ish%.

You're probably stagnating. Tempo runs are a great tool to use in training, but they are just that. One tool. You need a variety of tools to improve your fitness. It sounds like, based on your PRs, that you are doing marathon pace tempo runs, maybe even slower. Something to try to implement could be some faster aerobic repetitions, say at 5:45-6:00 pace. You can do this by time or by distance. Or maybe you can include some speed work at mile pace or faster to increase your running economy, in say 200-400m repetitions. Maybe even throw in some continuous fartleks and progression runs too. A few strides at the end of easy runs. Run up and down some hills. Give your body a chance to adapt to new training stimuli. Maybe then you will see some improvement!