We can easily look at similar examples, even for very severe cases, such as for prolonged immobility and coma. How do people rehabilitate from months and years of coma? I would assume it's a rather straightforward process to return to normal function. I'm pretty sure that in 90%+ of cases there is zero need to do anything, people will just naturally go for it without a hitch. And I'm probably underestimating it, could easily be 99%.
There are examples from, IIRC, the encephalitis lethargica cases that followed the Spanish flu pandemic, when someone found a treatment that worked in a manner of hours, but unfortunately did not solve the problem, only worked temporarily. Some of them had essentially not moved for years, and many started dancing.
Frankly all this deconditioning is half fear-mongering, and half a poor excuse to justify their professional failures. If the problem is entirely lifted, zero PEM, just the effects of years of extreme sedentarity, the vast majority are back to almost normal in 2-4 weeks, running, dancing and doing most of the things they want to do. Not running long, not fit by any stretch, in fact, badly out of shape, but capable of doing a normal routine for their age, and accounting for other health problems they may have.
Most of the problems will be more about aging, when it's a lot harder to return to normal. For that it may require some support, but looking at how inept the rehabilitation industry is with us, I can't see them credibly having a useful role. It would probably take them far longer to learn to stop doing most of the mistakes they will commit at first than it would take for almost everyone to get back to normal.
The hardest part by far will actually be to reach everyone with the treatments. To build the clinics and expertise that can deliver them, to find the patients, especially those who have become completely socially isolated, to get them to where the clinics are, to fund the programs that will pay for the treatments, and so on. About 99% of that work happens before someone gets the treatment. The rest will mostly be a breeze.
Well, the biological anyway. The socioeconomic, though, whew. Some of us will have never worked a day in their lives, not even finished school. Even a 10 year gap on a resume is a hard thing to overcome. I wish I only had a 10 year gap. Some people will have worked in careers that no longer exist, or would need total retraining. Maybe they wouldn't want to do that again, and would need new training.
This will likely be far more expensive and difficult to deal with, and a lot won't quite be able to make it through. Especially those who find themselves able to function again, now elderly, and with no one at all in their lives. No spouse, no family, no friends, everyone gone. That is likely something we will have to do ourselves, build communities so that we can rebuild meaningful lives with the support of the only people who truly get it.