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Humans have a Lactic Threshold (LT threshold).
At a power level / Heart Rate below this threshold, the heart and lungs provide the oxygen required to power the muscles.
You will find you have good endurance below this LT Threshold.

Once you go above your LT Threshold, the Heart and Lungs can no longer provide all the Oxygen required, then you burn glycogen in the muscles.
There is a limited supply of stored glycogen and you can run out quickly.

I personally found at 80W I had good endurance. But at 100W I could totally kill my legs in just 10mins.
FTP Ramp tests indicated I should be able to go for an hour at 140-150W.... No chance!
It worked like this for months of self training. In the end I hired a cycling coach.
Coach found the issue... my diabetic wonder drug was the cause.
It was suppressing all the glycogen storage in the muscles.

YMMV

Barry

BTW - not knowing further details, it's possible for your age and weight 80W is a respectable output.
In my case 56yrs & 200lb this was not the case.
There are FTP calculations, than given your personal info will provide you an estimate of expected output.