Dude, when I first started out, and I'm talking after not riding for 10-15 years, I got a mountain bike and pedaled to work every day (about 5 miles each way with lots of short 5-10% grade hills).
After the first week, my legs were perpetually fatigued. I could hardly walk up stairs without my legs feeling like I was walking on Jupiter. Regardless, every time I got on the bike, I felt compelled to go balls-out until I got tunnel vision. Guess its just my competitive nature.
Anyway, after a month or two, the fatigue slowly dissipated and my legs felt "normal" again. A year later, its all history. People now fear me for my ability to "drop carbon" on my ultra-sexay CAAD9. And women love me for it.