My first century was the Spring Death Valley Century last year. I remember being very nervous and wanting to do no more than just finish. My longest previous rides had been 50 and 80 miles. I did it with a friend who has since become my regular riding buddy (and with whom I just completed my California Triple Crown with last weekend.) It was horribly windy with head winds for 90 of the 100 miles. By the end it was steady 20 mph gusting to 40. We were pedaling hard just to go downhill. We finished it in 8:29 with a rolling average of 14 mph.
When we were done I was addicted! Have been ever since.