I was pondering this earlier, and concluded that I just need to ride more. My commute is a bit easier, but due to a combination of
@buggi's explanation and me not having much time to do a lot of other riding (well, not as much time as I have excuses, anyway) it's not loads easier.
The way I thought about it was this - really unhealthy people probably get off the sofa and go upstairs at least a few times a day, but it's still hard work because just doing something 'hard' for a short while only gets you just fit enough to do that, and no fitter. If that unhealthy person went for a jog of a few miles twice a day, getting up the stairs would be loads easier.
And thus is it with cycling, I concluded. If I want my commute to feel significantly easier I need to do more than just ride the commute. (That said, the first time I did it my OH considered calling an ambulance when I collapsed in a wheezy heap in the garden - it had been 15 years since I'd ridden a bike. She tends not to do that any more!)