Can’t be arsed to read back over the last 4 pages so if someone has already made the same points please forgive me.
Firstly are you making use of the drops?
I only ask as by my own (Probably flawed) estimation a stint on the drops instead of staying up on the hoods can easily “Cheat” you an extra 1-2mph along a long straight (especially when coupled with a bit of a push and maybe shifting 1 or 2 sprockets at the back)…
The other thing is choosing when and how to push and where to pace yourself or back off to make the biggest overall impact and making use of all the gears rather than a select few, a minor 1 or 2 shifts either way as the road goes up or down helps maintian pace, just because it’s not an MTB doesn’t mean you can switch off and stop reading the trail (road) ahead…
I’d not pay much attention to peoples claimed “Average” speeds, someone might well manage 24Mph, but they might also be fresh as a daisy on a TT bike on a dead flat smooth road, where you are probably dealing with a mixture of climbs, Descents, knackered roads and traffic, all of which can dent or help a route time quite easily.
Plus don’t forget you are riding solo which will have a big impact, people might tell you they averaged 20mph+ but they might ommit the fact that they spent the majority of that time nicely tucked up in the back of a chaingang drafting their way to Stava glory…
The best indicator of how much effort you are putting in is how knackered you are following a ride, you are much better off listening to your body and not interweb bravado…
Happy trails.