Quality of sleep makes a huge difference to your mental faculties. If you could bottle it, the effect would be a multi, multi $billion drug.
Just because some people have conditions that prevent them from exercising it doesn't mean it isn't the healthy choice overall.
And a cursory look at the literature for PEM[1] will show you that the recommended path to recovery isn’t “avoid physical exertion at all costs”, but to do what you’re capable of and slowly ramp up the intensity, in which case small amounts of exercise could play a vital role.
[1] http://www.phsa.ca/health-info-site/Documents/post_covid-19_...
Can't say I feel any different, but it is enjoyable.
https://web.archive.org/web/20230609011808/https://old.reddi...
It's not perfect, but it was great for getting into the habit 3 times a week. Now I often make up my own sets as I go, taking note of what's sore on me and what I feel could use some work. Doing that gave me the time to do research and get a bit more familiar with how to engage certain muscles, and what machines will do what for me.
IMO: most of what makes fit people fit is becoming aware of their body in detail: how to use muscles, how they work, what they feel like, what they should feel like when they're working together -- all of this builds a foundation where, if they lost their fitness, they would just get it back again.
So if you don't know what you're doing: just learn by doing. Pick something that it seems like your body should be able to do easily, but can't, and fix it. Repeat x100 and you'll be fit as hell.
Not causing injuries is the hardest part.
You can start with bodyweight though. Pushups, pull-ups, etc. I recommend the YouTube channel FitnessFAQs or Calisthenicmovement for a good start there. No nonsense, direct guides with discussion of technique and injury avoidance.
Book some regular sessions with a personal trainer. He/she will give you a balanced basic routine and teach you good form.
I was fortunate to have an excellent PT when I first ventured into the gym age 19, a skinny, awkward, scared teenager. The structure of that gym program is still the basis of what I do 30 years later. Different exercises get incorporated, depending on my goals at the time, and variations in # of reps and sets. But still the same general approach.