Set a goal. I use marathons (half marathons, 5 or 10 k's) to motivate me to exercise. The goal is not to "win," but to DO IT. I just finished the LA Marathon on March 21st. 7 hours, 1 minute and 23 seconds! 7 hours of steady 16-minute miles. And you get a t-shirt! Start training early!
Once you pay the entry fee, it is throwing money away to abandon the goal. Once you get out there and begin to see results, you simply do not want to quit! Get others on board. Do it with friends and colleagues. Do it for a charity. Just DO IT! It is so much fun, challenging and satisfying.