We know that COVID depletes NAD+ reserves and NAD+ is the main electron transport molecule in the body (second one is FAD+ in some cells in the brain). So if you suddenly run out of electron transporter, it would inhibit most of your systems, making you tired all the time, and the systems that are already operating near their maximal capacity will start failing. This might kick off a chain reaction that is later transformed into condition we call long covid. It would also explain why it is different for each person as everyone has different organs in bad shape and why people with certain organ damage have similar symptoms.