There are permanent big group rides behind bots, and probably the most popular 'C' one goes at 40kmh on a flat road. I don't think it's that unrealistic though, I'm a very average rider and 38-40km is about what I'd do in a small group at HHV on road bikes over 2 x 30 mins.
How are people I know to be fairly average cyclists clocking (an admittedly virtual) 40km in an hour on Zwift on a regular basis?
I assume it’s because of drafting but even so I’d be surprised if they could cling on to a chain gang at that pace.