So i bought a new Specialized Roubaix 09. I upgraded the wheels to my Mavic Aksiums, the Brakes to Ultegra and the saddle. I love the ride quality, the bike is zippy and super smooth and about 3 lbs lighter than my old bike. So here is the part i don't understand.

I went on a 25 mile ride yesterday and my legs were really sore after the ride. It didn't feel like a fit issue because there wasn't any pain during the ride, just that they felt really fatigued. I haven't ridden that much lately (10-15 miles about 5 times a month) so it could just be that i'm really out of shape. My girlfriend went with me on her 20 year old super soft steel allez and had no problem riding that distance. She rides even less than me. I know that i need to train more and shouldn't beat myself up about it but i don't consider 25 miles to be strenuous at all.

So is it:
A. I'm out of shape and should just suck it up.
B. My new ride isn't stiff enough and i'm putting more energy into flex than movement.
C. Switching from Standard Double to Compact will take some getting used to.
D. New Bikes Take time.
or
E. All of the above.

Thanks