From NPR 1/24: A discovery in the muscles of long COVID patients may explain exercise troubles

Now research published this month in Nature Communications gives new weight to this assessment.

By taking biopsies from long COVID patients before and after exercising, scientists in the Netherlands constructed a startling picture of widespread abnormalities in muscle tissue that may explain this severe reaction to physical activity.

Among the most striking findings were clear signs that the cellular power plants, the mitochondria, are compromised and the tissue starved for energy.

"We saw this immediately and it's very profound," says Braeden Charlton, one of the study's authors at Vrije University in Amsterdam.

The tissue samples from long COVID patients also revealed severe muscle damage, a disturbed immune response, and a buildup of microclots.
https://www.npr.org/sections/health-shots/2024/01/09/1223077307/long-covid-exercise-post-exertional-malaise-mitochondria
I have had long covid since 9/22. What you are experiencing is what I call "the roller coaster". If I have am feeling good and I have a required activity with physical exertion, I plan on a "crash" within the next 3 days, and it does hit.
My wife and I walked one mile per day before covid. Now I am slowly up to 1/4 mile once a week, and than I wait to see if I will be hit with PEM.
I have found 3 food items helpful: natto (for the microclots) with kimchi (for the immune system) and manuka honey (counters the PEM ).
It goes against our nature, and our obligations, to rest in order to get better, but you will not get better with over exertion.