I’m also coming back from surgery. It was a shoulder revision in my case but I have a chronic infection and on maximal dose of antibiotics For three months.
(I can’t believe how much I sweat now. Maybe the infection, maybe antibiotics.)
After about 8 weeks of no more activity than walking my legs were visibly smaller. Im mid fifties so muscle loss is fast. Conversely I expect muscle build up to be slow.
My observations for you. Core strength is what allows you to put big power down. I’m not at all surprised you couldn’t get through the sprints. In my opinion you shouldn’t be expecting to yet. That burberg workout is brutal when at one’s best. Don’t get despondent about that horrible workout.
I would work on gently, very gently bringing up core strength and expect that to take months.
I’ve started back with Zone 2 work of no more than 60 mins. My experience was that to keep in Zone 2 for heart rate I had to keep power much lower than previously. I am prioritising Heart Rate rather than power for what I’m calling Z2. It’s hard not to stray into Z3.
Even after 2 weeks I’m seeing the power I can make coming up, whilst still keeping Z2 heart rate. I’m building from that.
And the Z2 workouts don’t add much fatigue so I’m doing every day.