Is it possible? So long in zone 5?

I would say no. Given that LT would be around a 15k to half marathon race pace. Above that would be in the VO2 Max range or 3K to 5K race pace.

How did you derive your max heart rate?

From fenix 5s

Perhaps my zones are too low?
Or May be I should use my testing hr as a base?

Is there a Way to somehow estimate lt threshold from the last 20 minutes of my run?

Tell me please - is it possible to run for about 30 minutes in the highest hr zone?
I was not slowing down (even running a bit faster in the end)
Is it because of wrong hr zones?
160 is my LT threshold - that's what the watch says.

Thanks for your help

30 min continuously you really shouldn't, but if it done on and off such as fartlek type running or intervals and done within a longer period of time then yes.

Your LT estimation is probably off, mine is quite a bit, Fenix5 says 162+/- and when tested in the lab it is 176, a very significant difference that would completely throw off my HR Zones if I would base them on the watch's estimate. And would give me the same problem you are having.

If you can't do a stress test on the treadmill to determine it, then I suggest you calculate your real MaxHR and base your HR zones on that, would probably be more accurate.

But as TheCrowning says, 220-age formula the heart uses is completely useless and unscientific, same thing happens to me,my MaxHR would be 174 according to the age formula when in reality is 190 in the treadmill stress test, again huge difference, my HR Zones based on the age formula would be a joke.

But you don't need a stress test for determining your MaxHR, you can do your own test by yourself, outside or on the treadmill, just search online for it, it is simple. That is why I say to use the MaxHR to base your HR Zones, because it is a value you can really determine by yourself, unlike LT. That or do a stress test in the lab and find out all your values (maxHR, LT, VO2max, etc).

I say to use the MaxHR to base your HR Zones, because it is a value you can really determine by yourself, unlike LT.

There is a LT test somewhere on the 5/5S you can follow to find your LT. However, much easier method IMHO is to do the following:

Warm up for 15 mins - include some 100m to 200m strideouts, then easy for the last 2 or 3 minutes.
Press lap. Run fo 10 mins at a pace you think you can hold for 30 mins.
Press lap. Run for 20 mins at the same pace, aiming to empty the tank at the end. The aim is to avoid fading at the end, but at the same time ensure you run as hard as you can and still push a bit at the end.
Press lap. Cool down for 15 mins.

Your LT heart rate is the average HR for the 20 mins. Your LT pace is the average pace for the 20 mins.

Your max HR might be right, and your lactose might be right.
But your definition of zone 5 is wrong. Zone 5 should be 10 % of your Max HR, as a golden rule. So 180 to 200 for you. Not from 160.

Your lactose HR is more an indication for the speed you can keep for 1 hour, or an half marathon if you are well trained

Zone 5 should be 10 % of your Max HR

Zone 5 typically taken as 90-100% of max HR. You probably mean the same thing, just looks a bit awry.

LT = Lactate Threshold.

One more thing....
There is a HUGE difference if I chcange my zones according to my resting hr
Just look at the photo :-)

The big difference is because one assuming a maxHR of 178 and the other 198. Just test your maxHR on a run and find out.

Zone 5 typically taken as 90-100% of max HR. You probably mean the same thing, just looks a bit awry.

LT = Lactate Threshold.

Yes of course :)
Bit typo on my part.
Just to mention Lactate Threshold and zone 5 are not really connected.