Be aware that people are wont to ride in lower categories than they are supposed to and that, bizarrely, there isn't a mechanism in Zwift for dealing with this except to disqualify them after the race. C and D races are particularly prone to being infiltrated by B (and sometimes A) riders with fragile egos, which completely disrupts the flow of the races, and certainly is a major cause of the aforementioned frantic starts. Don't let this get you down! For example, this guy rode as a D last night...
View attachment 616065
Some races are for all categories starting together, while others have staggered separate starts for the different categories. I usually prefer the former - I'm a lower-end A rider and it's good to benchmark myself against the strong B riders rather than facing the prospect of coming plum-last in A-only races!
There are time-trial events, too, mostly on the pan-flat Tempus Fugit course and the love-it-or-hate-it hilly Bologna course. If you can get your head in the right place to hurt yourself for 11 miles, Tempus Fugit will give you a good opportunity to improve your FTP, since time-trialling is about putting out a constant, sustainable effort, rather than the constant surge-and-lull of mass-start events.
Look for well-attended events - it's crap when you do a race and find it's only you and one other on the startline!