It's going to vary depending on the course, but you're a little off in your estimations...for a flatter course of Oly length (~25 miles), pros average around 27-29mph from what I've seen, that's why they're pros.

If you're breaking the 20mph avg pace as a guy, that will put you usually in the top 1/3 depending on the competitiveness of the race...however it might be the lower end of the top 1/3 for Oly's.

Also, 3:30 miles is just over 17mph (17.1ish) just for reference...if you have a Garmin 405 I think you can set it to read mph pretty easily.

All that being said, your race speed and training speed can be vastly different depending on traffic, stop signs/lights, etc...so unless your 25mph ride was on a closed course or very isolated, you likely will go substantially faster in a race. For reference, most of my road bike rides (hillier, traffic) avg around 17-18mph for 30-40 miles, my tri-bike training rides (bike path, flatter, more isolated) avg 18.5-20mph, and I race sprints and olys in the 22-23.5mph range and did an open 40k TT with a 23.9mph average recently. Hopefully that gives you an idea of what to expect

To improve...ride more, ride harder, ride varying terrain...all have paid off big for me w/o investment in race wheels or powermeters to this point