If you visit Cycling Clubs websites, they have race reports and results, but don't publish times and speeds, except for TTs.
My best advice is to join a club, talk to their racing coach and enter a Cat 4. See how you perform.
As for 'trade jerseys' Pt II.
A newbie on a midrange bike wearing full team replica clothing WAS known as a 'Wanabee'. An old guy on an old bike wearing old trade clothing WAS known as a 'Nevawozzer'.
If you see an oldish bloke on a Peugeot 531 team PX10 wearing a merino Pug jersey with world champ collar and cuffs, its me the 'Nevawozzer', not Tom Simpson's ghost.