Like Tomsk said, take it slowly. I had COVID just under a year ago and it’s been a difficult road back to feeling confident on the bike. If I pushed myself too hard too quickly, I’d get a COVID flashback, which in turn would dent my confidence and lead to a downward spiral.
After a few false starts last year, after which I never thought I would feel normal again, I decided to take tomsk’s advice and ease myself back in very slowly. Starting more or less form zero at the beginning of the year, I kept the distance and intensity very low, but did it frequently. I prioritised eating healthily, hydrating properly, threw in some yoga and walking, and most importantly listened to my body.
I’ve built it up over the last 2.5 months and am now doing 6 cycling sessions a week - 3 on the indoor trainer and 3 outside. The indoor trainer sessions allow me to increase intensity safely and the outside sessions have been about increasing volume, but all done at 3-4kph less than I would usually ride a brevet.
The result is that I’ve managed to raise my FTP from about 190W to 265W and last weekend I managed to ride 135km (I’m working my way up to 200k by the end of this month). I still don’t feel great on some days, and there is certainly an anxiety that I will regress, or long covid will strike again. But the important thing is that I’ve enjoyed the journey. At the beginning, I was worried, and almost convinced, I would never be able to ride a 200 again. But through a very gentle, but consistent “rehab” programme, I now feel that a 200 is within my grasp again.
Good luck, but take tomsk’s advice, and start small and go gently.