I'be been just skimming through some of the comments.

Like RoadRaven says, twice the srength doesn't make you go twice the speed. Speed and power is not one for one. To go twice the speed, you need about 4 times the strength/power.

So when you first start riding, going from 10mph to 15 mph doesn't take much. trying to improve from 15mph to 20mph will take a lot more. And if you want to go from 20 to 25MPH, you really need to train super hard and do things you don't want to.

It wasn't all that unusual for me to be chucking biscuits* on hill reps years ago. But that's how hard I needed to push to get my speed up to 26mph for two hour strech.

And there really isn't a substitute to hard training.

the other thing to remember about training on your own is that we all tend to work out on areas where we are good at and sloff off on areas we are not. This is where having a coach really helps. They make you practice and work out on your weak area.

If you are having problems on hills you need to do hill reps.

sorry for the bad news
smilingcat
* pull over to the side of the road and hrowing up all the meanwhile you keep cranking away