Spin Legs

Where have my spin legs gone?  Now that I've been mountain biking for almost three weeks, my spin classes seems so hard.  My legs feel so tired.  What's going on?  Is mountain biking that easy?  Is spin class that hard?  Am I over-worked, or just so out of shape I can't do both.  Either way, my pants fit better and I'm having fun.  So I'll enjoy it while I can.  I have a real job interview tomorrow....yikes!