I got COVID towards the end of August. It laid me flat for three days. I was sleeping for 12-14 hours a day.
That first week saw me do nothing. NOTHING. The most I could do was make it down stairs to sit in the garden and feel some fresh air on my face. Honestly, training and FTP were the last things on my mind. In total, I took seven days completely off the bike.
In the second week, I started easy with Petit. No Spin-Ups or Form Sprints, just spinning the legs. I kept the cadence low and focused on my breathing. My HR did bump up to my upper endurance levels but it would flatten out and remain level for the duration of the workout.
After a 3-4 days of Petit and the-like, I headed outdoors (once my isolation period had passed) and did more easy spins. No more than an hour and a half at most.
Second week I tackled West Vidette +1 and Carson +1. My legs were there but my breathing was still towards the top end of my zones so I’d consciously back things off.
At the end of the day, you have to listen to your body. I had a rough three days but in all honestly, I think my COVID experience was very mild. I had no respiratory issues. It’s also going to depend on where you were in your training. I was in the final six weeks before my A Event. I was carrying good fitness.
My advice. Take it easy. Very easy. Like coming back from the off season. Give yourself time and be prepared to adapt. I dropped my FTP by 20 watts. I gained 10 of those back in six weeks but I did have to approach my A Event with an open mind.
Good luck moving forward 