The good thing about marathon walking, as opposed running or even racewalking, is that you've been doing it your whole life. If time/pace is not an issue you will probably be able to just head out the door in a pair of comfortable, supportive shoes and start walking. If you're currently able to stay on your feet for an hour of shopping, you should be able to walk for an hour which should add up to three to four miles. After that it's just a matter of adding more. Most days you can just get out and walk an easy 45-60 minutes; one day per week you should build your distance gradually--about 10% per week--to build up your endurance for the marathon. You may also consider pushing your pace a little faster on one or two of your mid-week walks.
I have a beginning walk schedule posted here.
A journey of a thousand miles--or 26.2--begins with a single step. Take that step!