There's good advice here... especially making sure your bike fit is good.

I echo the suggestions to JUST KEEP GOING!

And yes.. once you make sure your bike fit is OK and you begin learning proper riding (cadence/gearing, etc.), definitely go out for longer rides. If you ride 10 miles, 3 is a cinch. If you ride 20, 10 is a cinch. If your ride 30... and so on. You do need those rest days though, as somebody else suggested.

When I started out I couldn't ride more than .1 mile without coasting, and a three mile ride had me whipped. It took awhile, but I gradually got better. I now ride 10,000+ a year and go out to ride centuries 'on a whim'. Not saying that you will or should do the same, only that what you think is hard now you will laugh about in the not too distant future.