I bet it's too much too soon.
Although I have not had knee surgery like the OP did, I was in the same position as the OP in that I was never an athlete prior to bike racing and I started about a year or two ago.
I started up with a coach and during the winter training program perscribed about 700+ TSS weeks combined with strength training / core training 3 days a week. I think my body just took a sh*t on me from being pushed too hard too soon (along with fit issues) so my left VMO started to ache every time I got on the bike.
This was starting early November and now I'm here about 10 lbs heavier (all fat I bet) and completely detrained. I started my new training regimine with a new coach and hope for the best, but like you my early season races I was planning on doing well on are out of the question.
