The Passive Hypothyroid Slump - how to stop napping

sounds contradictory: how can you have the energy to exercise but instantly fade when sitting still? There are a few highly specific medical reasons for this after thyroid cancer treatment:

The "Passive" Hypothyroid Slump: When you are walking or exercising, your body pumps adrenaline and cortisol, which masks your underlying hormone levels. But when you sit quietly (reading) or engage in a monotonous, hypnotic task (driving), that artificial boost drops. If your synthetic thyroid hormone (levothyroxine/Synthroid) levels are even slightly off, your body crashes into a deep, passive fatigue.

I can walk 10k steps. But if I sit to read or do anything....I fall asleep. I dont want to spend my day walking and taking 3 naps.
Does anyone have this issue?
Thyroidectomy october 2024 and finished chemoradiation March 2025. I am cancer free.
Thanks!