I ran my first half marathon at the Wine and Dine inaugural half marathon in 2010. Since then, I've done a bunch of smaller races, another half and a full marathon.
First, you're going to have good days and bad with running. Everyone does. Some days it takes all you've got just to get out the door. Anything is better than nothing.
Take it slow. It can be incredibly frustrating to feel like you're improving greatly day by day only to be sidelined by an injury because you're pushing too hard, too fast. I would greatly suggest reading Marathoning for Mortals. It is a great book with running (and run/walk) training plans in the back of the book. Also, it is very motivational to read other people's stories in the book.
Keep track of your progress. On days where you are down on yourself because you had to walk too much of your training run or you didn't go as far as you had wanted, look at your running journal and see how truly far you've come.
Read running blogs! I really enjoy reading blogs of people who do ultra marathons, Ironmans and the like, not because I am remotely in the same fitness circle as them, but when I have a 5+ mile training run ahead of me and I read about their grueling 25 mile training run in 90 degree heat, it makes me feel a little better and like I don't have it so bad

. One of my favorites is
www.shutupandrun.net but there's lots out there!
Like a PP said, enter smaller races. I did about 5 races (5k to 10k) prior to my first half and I'm so glad I did! It takes a lot of the race day nerves away when you're not pinning a bib to your shirt for the first time or dealing with corrals or things you didn't anticipate (port-a-potty lines, running in the rain, etc.), but most of all I think it helps to teach you to pace yourself with race day adrenaline and not getting burned out too soon (longest sentence ever).
Give yourself small goals. Like if you do all the training runs that you have on the calendar this week, you get a reward. It can be something like a meal at your favorite place, a new pair of running shorts, or watching your favorite movie that weekend. I find this really helps in the final months of training when you're logging longer runs and starting to get bored or burned out from all the running. Sometimes all it takes is a new pair of running shoes or running accessory to refocus on the upcoming race and my training.
At the end of the day, you will find the motivation inside of you. Don't compare yourself to others or let the voice inside your head bring you down. Go at your own pace and enjoy the feeling of finishing a long training run and especially that feeling of crossing the finish line. It makes it all worth it!