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 Walkingby 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!

Training plans are also available at:
Race Walking  racewalk.com

Dave McGovern Marathon Plan

YouTube Race Walking with Coach Ian Whatley

Minnesota Twin Cities Racewalkers

Whatever training approach you choose,  walk and have fun!