That's it right there. I've been commuting for about a year now and my legs still burn, and I'm puffing like crazy going up the same hill I always go up. The only difference is I'm going up a helluva lot faster than when I first started commuting.

Oh, and the constant never-ending headwind I experience biking home from work pretty much guarantees it will never ever ever get easy, regardless of how in shape I get.