1. Find A Race.
On any given Sunday, there are often thirty or more half-marathons available to walk in the United States. Here are a few sites to help you find a race, near or far, easy or hard. Visit each half-marathon's official website for additional details.
This site is my first choice in finding half-marathons in the United States. The list is comprehensive, constantly updated and includes tips on training and "best of" lists. If you are an avid half-marathoner, you likely visit this site daily.
This is a go-to site for worldwide listings of half-, full- and ultra-marathons. It takes a bit more work to navigate, but it is arguably the most comprehensive listing of races around the globe. Race information is fairly comprehensive.
Active.com offers online registration services for many races. Their lists are not as comprehensive as halfmarathons.net or ahotu.com, but is still a great site to browse for upcoming races.
I've walked several half-marathons with friends of varying degrees of both fitness and preparation. Each one finished their race. The only difference between the one's who trained and the one's that didn't is how they felt the next couple of days.
There are several sites with great training plans. The bottom line is that you should walk at three-to-four times a week for several weeks before your event with several longer distance walks once-per-week. When I first began racing I stuck to a fairly rigid program of walking Tues, Wed and Thur for 3-5 miles each. Then I'd steadily increase my long weekend walk from 6 to 10 miles in the weeks leading up to the race. By my ninth race, I knew exactly what my body needed in the weeks leading up to a race. Now, I maintain a daily 2-4 mile walk on weekdays and 6-10 mile walks twice each month.