+1 for getting used to it, I found the same thing when I started riding every weekend day for my commute. Also eat something with protein in it immediately after each ride, I have a box of Almonds / Pumpkin Seeds and raisins at my desk at work which I munch a bit of every day.
And finally as you get fitter you will find you can ride within yourself more easily and not get so tired, but still go faster than you used to :). And suddenly the weekend rides are more fun as go faster! Result! 🙂