Pretending You're Too Busy
This is the number-one excuse for not exercising. But your buddy with that annoying 10K habit is just as busy as you are. The real barrier is lack of consistent motivation. Or as ultramarathon man Dean Karnazes puts it, "Where there's enough motivation to exercise, there's always enough time to exercise."
The Fix: The easiest way to get motivated is through competition—that's why CrossFit, which keeps a score for every workout, has been so successful. Sign up for a century bike tour, marathon, triathlon—anything. Then find a partner (the alarm clock might not make you get out of bed, but a waiting buddy will) or join a training group (try Active.com, a social-networking site for athletes). Or just think about how you'll feel after exercising. "Not once have I felt worse after finishing a run than I did before I started," says Karnazes.