Personal anecdotal report. I was relatively weak and fat in HS. During college, I had a delusion of applying for Marine OCS and started training to their PFT of the time. I ran, and ran a lot. I started going to gyms and training with some fairly tough athletes (not body builders). I used to max the LE PFT (Cooper test) which as far as I know was first used seriously in Illinois in the 80s.
Part of the training goal was of course prevailing in fights, and conveying the impression that fighting me was a really bad idea. I was pretty muscular - I had to take off a dress shirt to get a shot because I could not roll up my sleeves. I had to stop running after a weird foot injury, but I already had exercise bikes. I have broken most of them and am on my sixth which has been repaired several times. My typical CV training on the bike involved at least 30 minutes and up to an hour of getting to 90+% of predicted max heart rate. I sweat so heavily that I had to change shirts twice to go the gym. In-service control tactics training was ... interesting and I was told by a couple of assigned partners that I was too strong to overcome one on one.
My kidney problem was first diagnosed when I was almost 50. The initial treatment tested my pancreas and it failed, so I became diabetic. Eventually that was controlled pretty well, mostly exercise, some diet. My kidney failure progressed to the point that I was being screened for transplant. Part of the testing involved a cardiac workup that resulted in stenting then an eventual bypass. My ticker was fine, but some of the arteries were not. Likely genetic based on family history. The recovery from that, a GI bleed, and then hernia surgery cost me a lot of conditioning. I do what I can, but recovering has been ponderous. (My regard for the various medical folks is very high; they have taken good care of me.)
My appetite has decreased, my power and weight along with that. My dry weight for dialysis started at about 220, I am down to about 190 now. Within the realm of people in need of a transplant, I am doing pretty darned well. I do what I can to make it work. Investing in your health is part of the battle. I too have seen food blisters at classes who not only could not do what they needed to, but presented a risk to others because of their performance problems.
If one looks at the general advice about fitness training, much of what is recommended is ok for people whose lives do not depend on fitness, and is designed not to intimidate them. A more detailed bit of personal research will lead you to a far more vigorous program. Is this a bit harsh? Yes. Does that make it untrue? No.
My genetics are to some extent crummy. Overcoming them matters. Your body, your choice, but reality is a harsh mistress.