Motivation is required from Start to Finish

Get someone to hold you accountable. Whether it's a real-life person, such as a spouse, a parent, or a friend, or someone online like a blog audience, give someone the authority to yell at you when you are slacking. Make sure it's someone you trust and respect, because otherwise their words will go unheard by you. Tell them to be brutal. Tell them to be persistent. Tell them that no matter what you say, no matter what excuse you give, they are to badger you relentlessly until you get off your seat and exercise. Even if you only do it out of spite, it doesn't matter, because it's still exercise.
Make sure, of course, that the person or people you choose can handle a little backtalk, because you will talk back. There will be a day when you really, really don't want to do anything. But those days do not represent your life or your future. Those days are anomalies and don't happen often enough to decide the rest of your life. The decision you're making today to change your life for the better is your default setting. Someone who cares enough about you to kick your in the rear from time to time will understand what you're trying to do, and will be doing it for your own good.
Join a club. Whatever it is you decide to do, find other people who also do it. This is another way the Internet can help you out. No matter what activity you settle on, no matter how strange or unusual it is, there is an Internet community out there devoted to only that. Finding other people who do what you do can provide a great sense of accomplishment and unity with the world. When you don't feel alone, you feel more motivated to do what needs to be done.
When you're feeling frustrated, having friends who have been there can be invaluable. You don't even have to say anything. Just being there, being around people of a like mind, can boost your spirits tremendously. Of course, a community is even better when it's a "real life" one.
If your city or neighborhood hosts a team or a club devoted to your activity of choice, join up immediately. You want to surround yourself with people who will encourage you to achieve your goals. Being able to call someone up or meeting someone for coffee and venting can work wonders.
Of course, the really amazing part is when you become the motivator for someone else. One day, you'll be the one to say just the right thing to someone who is in your position now. You'll be able to think back to this day, when you felt unmotivated and scared, and give them real advice based on real experience, and it will help them the way your mentors help you now. And nothing could be more fulfilling than becoming a role model for someone else who wants to change their life.