There are many ways to train for a walking marathon or half marathon. You should select an approach that is appropriate for you and your situation.
If you are strongly self motivated, and want to train on your own, the book The Complete Guide to Marathon Walking by Dave McGovern would be a good choice. It is an excellent reference for long distance walking, and covers both marathon and half marathon distances. Available at amazon.com, or directly from Dave McGovern
Some athletic clothing specialty stores (for instance The Running Room) have structured training programs for walking a marathon or half marathon. Check with the stores in your area.
Many people find that they train more regularly with a group. The support from a regular group walk is impressive. Organize your own training group!
YouTube Race Walking with Coach Ian Whatley
Minnesota Twin Cities Racewalkers
Whatever training plan you choose, walk and have fun!