You also have to keep in mind that your weight might not change. If you are already thin but not in shape, those pounds might not come off. I have always been thin as well, but not in shape. I bike 20 miles a day to and from work and then put about 100 miles a week on my other bike. I eat fairly well (I slip now and then). I weigh exactly the same now as I did in February when I started. The difference is, I'm mostly muscle at this point, especially my legs. And my clothes do fit better. It took a while though - it probably wasn't until about 5 months into it that I really noticed any difference at all.

Good luck and be patient, it'll happen