Post Viral Fatigue Syndrome

As someone with ME/CFS, stop fighting it. Listening to your body and just resting, completely, is your best bet at meaningful recovery. Many people miss the window of opportunity due to delays in diagnosis. If you are able to take time off work, do. Fourteen hour days aren't doing you any favours (I realise this is easier said than done, but time off now may mean you are able to get back to full time work. Two years down the rabbit hole I can still only work part time, and that's only relatively recently.

I would second eating at maintenance, at least for a bit, but do keep a very sharp eye on good nutrition.

Ignore anything you read about Graded Exercise Therapy. That works for some fatigue conditions, but if you do actually have ME/CFS, it's about the worst thing you can do (GET, is where you push to increase exercise, and are meant to keep pushing regardless of how you feel). Pacing (all activities, not just exercise) is the way to go. That can be really hard to learn, because when you feel better, you want to do more, and it just ends up setting you back (this is known as post exertion malaise - PEM). You'll often see recommended to try to find your 100%, then only do 80%, leaving the extra 20% in reserve for recovery. Couple of links on PEM and pacing for you:

https://m.e.awareness.nz/an-explanation-of-pem-for-pwme

https://m.e.awareness.nz/a-guide-to-pacing-for-pwme

Honestly, the thing that helped me the most was just letting myself be sick. Financial nightmare, but I have a level of recovery that if I got no better from here, I can live with. I still have to be extremely careful (a bit of a flare going on at the moment, for example), but it's a grand improvement on where I was.