Rest and exercise.
Resting restores energy, exercise helps you slowly build up a "larger capacity" for energy.
Aside from that, time. You may never fully recover the energy you used to have. We also lose our ability to store and maintain our energy with age, too. Loss of energy is a natural part of life.
I think that is a bit too gloomy, to be honest. Most of the people I know who, like me - and apparently like
@Psalm23 - had it mildly, have recovered fully after a month or two.
My experience was almost identical to that of a 25yr old woman who worked in my brother's medical practice. Both of us felt fine, would get on the bike to cycle to the shops and back - and would then be laid out, with no energy, the following day! This went on for a fortnight or so and gradually diminished. (I lost my sense of taste and smell for several weeks as well, but that's another story).
The people that get long term effects seem to be generally (though not always, admittedly) those who have had it badly enough to go to hospital, i.e. where it got seriously into their lungs.