Slomo, I have to admit that it is not easy. But, thanks to the internet, you can find a lot of info in several forums! It takes time, a lot of time. But at the end of the day, you will be able to make your own routes, or use ready made ones, downloaded from the net from peoples who shared them.
I don't know what kind of GPS you have, but Garmin is (for now) the only, able to "construct" your own trip.
Give it a try, and start reading reading and reading. That's all I can say.
Next summer we are heading to Scotland. I will use routes of friends who are there now. They will send me the files, I eventually modify them a little, and than store them in my SatNav.