Hi,
In some ways I am similar to you, and was a donor some 8 years ago. I am 62. My creatatine and GFR bounced around in the 1.5 area for creat. And between 45 and 50 GFR. I was a runner for some 30 years prior to being a donor. After donation I was tired a lot, became a walker, worried a bit about it being the kidney. Wondered if the donation was causing the tireness.
But I guess that is about as much as my story can help you, as about 2 years after donation I became dizzy, tired, unsteady on my feet, a little mixed up. Somewhat it did not help that I continued to have doctors look into the kidney function when I should have started sooner looking at other factors. After two years of testing the major findings were, B-12 deficiency, and not some scary neurological conditions like MS, dementia, and brain tumor. But it progressed, and about 2 years ago I was tested and Dx with neurological Lyme, cronic. Further testing found the both MTHFR gene mutantions, very low adreanal function which I am now getting some help with. Having the low kidney function has complicated treatment a lot, as I cannot easily get rid of the Rx I need to take, as well as the toxins created by treating the infection. I would look into if you have the MTHFR mutations, about 10 percent t of the population has this and if you are part of that group it can perhaps be a factor in treatment for your tiredness. I would also get the adrenal function tested. Best wishes. I hope you improve.