The biggest thing is to do faster bits of riding. If you can go 35mph on your own, even for a short time (30 seconds) then going 19 mph won't seem so hard. If you struggle to maintain 25 mph for a minute then 20 mph for an hour will be tough. The best way to improve your speed is to do group rides or races. If you can't do either then you can work on it on your own. It's much better to do it in a group though.

For solo speed work:
http://sprinterdellacasa.blogspot.co...sprinting.html
It's sort of like doing intervals without calling them intervals.

Another thing - as you got more cycling fit have you adjusted your position to reflect your stronger gluteus, back, shoulders? Position on the bike is a significant variable in overall drag of the bike/rider unit. Even an experienced racer might find a more aero position on their road bike, like Bernard Hinault towards the end of his career (he fitted a longer stem and promptly soloed to a victory in the Tour of Lombardia). As pros have more time in the wind tunnel they start adopting some interesting things on their bike - narrower bars, very low front ends, etc. Their overall power is not going to change much but their position can make a pretty big difference in sustainable speed.

Finally if you do the same thing over and over you'll just get good at doing exactly that. Doing the same ride all the time won't let you make the step up to the next level. Ride much harder on one day and then go much easier on another. If you want to bench press 150 lbs 10 times but you can only bench press 100 lbs 10 times you don't just bench press 100 lbs all the time. You bench press 120 lbs maybe 6 times or 140 lbs 4 times. Then you recover because you've pushed yourself beyond your normal comfort zone. Soon you'll be doing 180 lbs 4 times or 200 lbs twice. Then you can "move down" to 150 and rip out 10 reps.

Likewise on the bike you need to do much more intense riding sometimes, much less intense other times. Do bursts of 25-30 mph (or higher if possible - it would be great for you to be doing 30-35 mph for 30-60 seconds at a time). Repeat. Use downhills to build speed then maintain it on the following flat roads. Etc. Then on the easy days go 12-14 mph - you should be restraining yourself the whole ride. Save it for the next day when you ride hard.

I may be a long time racer but I almost never train at speeds over 18-19 mph avg. Most of my training, and I mean 95% of it, is at 15-16 mph avg speed. I'll do jumps to 38-40 mph, I'll draft behind a truck at 45-50 mph, but when I'm rolling around I'm doing in the 15-17 mph range. The last time I tried to go hard for an hour was a time trial forever ago and I averaged 23.5 mph, about 5 mph slower than my competitors Still, though, if you're going 18 now then you'll easily go 20 average if you mix it up some for a bit.