I would def see a good Doctor.
In my early 40's, I could barely ride a few blocks without immediate lactic acid build up. I went to the Doc and said, "my mitochondria are broken" and he tested me for lead and it was sky high. Doc treated me over some months and I eventually got better. Fortunately, he had had an olympian shotgunner who had this problem and he connected the dots for me. Possibly luck that he recognized it. OTOH, it is not normal for a young man of 53 to have such a drastic change like that. Unfortunately, OP probably will not get help easily but should pursue. Not normal. It isn't like the intensity and volume are huge or that he ramped up quickly. If a week of rest does not resolve the intense tiredness, something is wrong.