I think you’re on the right track here, along with
@moodleman who suggested Type 1 diabetes. I am a physical therapist and have worked with two men with Kennedy’s Disease in my career. Typically speech would be noticeably affected for a very long time, at least 6 months to 1 year or more. It is very unlikely (IMO) he could hike 10 miles/day and then so rapidly decline to death from Kennedy’s. Men (I believe women are only carriers) typically live to be elderly before succumbing to Kennedy’s, as it does not shorten life expectancy.
But as you and others noted, these types of diseases are often misdiagnosed in life, and would be virtually impossible to diagnose 2-3 days after death with decomposition (although I do not pretend to be a forensic pathologist, by any means). Even if we learn his identity, unless he told someone his diagnosis, we will likely never know.
I do think disease is much more likely than animal bite, ingesting plants or tainted water, or even eating disorder leading to fairly quick starvation. I believe if he had a long-standing eating disorder, he would show much more chronic damage to his body. I have reviewed the postmortem reports and I have no reason to think the ME would ignore significant chronic organ damage or even damage to his teeth from chronic malnutrition. Although he was reported to eat candy frequently, so he obviously had exceptional teeth.
My personal guess would be Guillain-Barré syndrome. I have had patients catch a minor cold, develop Guillain-Barré and become paralyzed on a ventilator within days. It can also progress more slowly, allowing for extreme weight loss before full paralysis. Without treatment, it would very likely be fatal and he would not be able to call for help easily (extreme weakness of respiratory and muscles of speech). I cannot imagine (again, IMO) the ME did nerve studies of our hiker’s myelin, if that is even possible after decomposition. It makes me incredibly sad to think of the manner in which he died alone, no matter the cause.