Retraining the mitochondrial cells

So i was speaking to a family friend whose son had CFS after glandular fever back in 2008. He got referred to a NHS program in bath, where they slowly increased the amount he did each months. it took him around 3 years, but he slowly but surely regained his capacity this way. He described it from what I understand, that the virus or stressor event caused the cells to use an inefficient method of generating ATP. Therefore this gradual increasing of effort (physical or cognitive) slowly gets them using the efficient method again. Is there any truth in any of this? I’m just looking for answers and how I can slowly start to improve, because I feel I have a baseline, it’s just so restricting and hard mentally when everyday feels the same. thanks