A few weeks have passed so I wanted to add a few more thoughts for anyone interested.
I've read Drew Baye's ($25) ebook "Project Kratos" which is a detailed high-intensity workout program using bodyweight exercises, most of which can be done without any equipment which is far more suitable for sick people who need to stay home.
http://baye.com/store/project-kratos/ Drew was a competitive body builder when he was young, but is now a family man and personal trainer. Like Doug McGuff I like his focus on safety and efficiency.
@SOC One of the things I learnt from Drew's book is that for each exercise he describes in detail how to do around five variants of it ranging from easy to hard. So for example I was finding that to do squats until my leg muscles failed to jelly was taking over 5mins, which contributed towards a weekly exercise duration that was getting far too high. I was getting sicker. Drew says with squats to make it harder by focusing on the lower half of the movement - only ever go half way up, and when at the bottom stay there for a few seconds. It makes the exercise much harder, but much shorter - for me at least that helps. My squats duration is now down to 2mins a week, but it still achieves the goal - to reach momentary muscle failure (burn, panting, weakness) which is what triggers biological adaption. A downside, as I've said before, is these exercises are not fun, it always by definition ends with soreness, panting and weakness. I often end up lying on the floor afterwards unable to move, utterly breathless and wiped out. My saviour appears to be the tiny exercise durations from which I recover. I still can't comprehend how a bit of gentle walking makes me so sick, but this sort of very high intensity exercise does not.
I neglected to mention in earlier posts that there are two other things I do that MAY be contributing to my ability to do this exercise (which currently totals around 15mins a week, spread across Mon/Wed/Fri). First, I've been taking 4mg of Naltrexone daily since 2011. I recently hit a supply problem and ran out, which has resulted in a stark reminder of how much sicker I am without it (esp far, far more malaise and "sickness"). Second, I hadn't realised until I started testing my ketones again recently, that my diet has unintentionally become somewhat keto. I do not deliberately eat a ketogenic diet, but by avoiding all the refined and sugary carbs and focusing on fish/meat/veg/fat I have ended up in the same place.
I've been watching a lot of videos online of experienced personal trainers talking about exercise and fitness and it seems that good fitness is less about cardiopulmonary health than it is about cellular metabolic health. I thought fit people had low resting heart rates because their heart was strong, but no, it appears to have just as much if not more to do with the fact that they have trained the metabolic processes in their cells to be more efficient, so the heart works less hard. My working theory/guess is... If I am fatigued at least partly because my cells are metabolising macronutrients inefficiently, can I train my cells (esp in muscles) to make up for that?
The question then is, what is the most efficient way to train cellular metabolism in my cells and mitochondria in order to achieve this possible improvement in efficiency? The answer (according to studies I have not read) is to use muscles to do work, to take them to momentary muscular failure (ie exercise them until they shake, weaken, and cannot physically do another rep) and to do it at such a (safe) intensity that this happens very quickly to reduce PEM. The muscular failure then causes the body to adapt, increasing efficiency, strength and so on for next time. I gather Doug McGuff works out less than 10mins a week, he is fairly muscular, and has a resting heart rate of 48bpm.
Interestingly McGuff and others are saying some people need 7-14 days between workouts to recover, and if they work out more often they get sick, exhausted, or just get no strength gains. If that is the case for healthy people us lot probably need more. I remind myself that the gains from exercise do not happen during the exercise, they happen in the days afterwards, while we are recovering. I do each exercise once a week, but may extend that to 10 days in future.
So I'm now (after nearly three months of exercise 3x a week) in the surprising position of doing a little too much exercise per week, so am now learning more about how to make the exercises harder (higher intensity, not speed), so I can get the muscles to fail quicker and shorten the duration from 15mins a week down to 10mins a week. I am enduring some PEM, but it is not cumulative. The malaise and "shitty feeling" is worse (I guess inflammation after exercise), but my balance is better, I am less weak and unstable, my energy level is very slightly better, daily activities like crouching down are far easier, I suspect my blood sugar control is better (less waking starving, less getting angry when hungry, less mood swings etc) and it seems my walking range has increased a little. I've also noticed my OI symptoms have improved - eg I now longer get dizzy when standing suddenly.
If anyone is interested there is a Dr McGuff talk here where he goes through his approach, and the science (to an audience of young men):