Expected time in cycling race
Ballpark would be ~2 hours, possibly down to 1 hour 45 minutes.  If you set a mental goal of "sub 2 hours" then you'll be on track.

You will be astonished by the "wind suck" produced simply by being on the road with a bunch of riders all going in the same direction.

You don't have to draught/draft and if you've never tried before then a large mass of people is utterly the wrong time to start.

Given there are thousands of riders, it sounds like more of a mass participation event than a race.

If you know the route and have time in the weeks leading up to the event, you should definitely ride the whole route.  This will teach you the corners and points to look out for.  However your time will be increased by intersections and road traffic.

If there are climbs in this route you will want to try them at various powers and see if your physique and endurance allows a high effort, or if you need to rein it in and get to the top with reserves for the next piece.

There's a good chance you'll find other people practising the route too.  So observe them and see if your actions are different.  Try it their way and see if it feels better.   Also this is the chance to set some strava times for segments.