It does get easier. There is a saying around these parts that "it doesn't get easier, you just get faster" which certainly has a ring of truth to it but on the whole it does get much easier.
Principally its a psychological thing. As you get more used to racking up the miles, you'll learn to pace yourself and manage your energy levels better, learn to read your body, your post-ride recovery will get quicker and easier, and ultimately longer rides become easier off the back of the fact that you'll already have x miles under your belt already. You get used to being knackered, bonking, aching legs - they're all part of the 'fun' and its how you react to them mentally that influences how you ride. Its merely an experience thing.
I've always said going from 50 to 100 miles is all-round easier than going from 10 to 20 miles when you are first starting out.
Stick at it.