Sometimes we all need a little extra motivation to get up and go, a great way to keep you going with your fitness regime is to have a definable goal. It isn't always enough to stay motivated by simply wanting to lose weight.
Tons of charities sponsor races and fun runs for worthy causes, so do a little research to find out what's happening in your area a few months down the line, pick a cause and register for the event.
Once you've made the commitment, train to do well in the event. Start by timing yourself on a run of the event's distance, then sit down and decide how much you want to improve by the date of the event. Now you're not only helping yourself get in better shape, you've got a worthy cause to go along with it and you're more likely to stick with your training if you've already registered for an event.