Ok, just one last number to compare - TTT at the Tour. 67.5 km (42.2 miles), winning team (Disco, of course) did 1:10.39, for 57.3 km/h (35.8 mph). Last team (AG2R) did 53.6 km/h (33.5 mph). Aero bikes, the whole thing. The first 40 kms on the flat, with a tailwind; last 20 kms a bit more technical, some rollers, crosswind. So if your mid-week rides maintain 33 mph no sweat on 40 miles runs, there are a lot of pro teams waiting to sign your group on.

Mind you, from what I read here, a lot of the "group rides" being mentioned in the thread actually seem like high-level amateur/semi-pro training sessions, and that kind of pace is quite possible at that level - perhaps not over 40 miles, but certainly over some extended periods. Not quite what one would expect of the local bike shop "let's get together on Wednesday night" ride, though.