I like taking long walks, although for me "long" is about an hour. I do notice the difference between muscles used for cyling and those used for walking, even with a walk of that distance. So you will need to do long walks for training.

Also, good shoes and socks will be important, and you might need to experiment a bit to determine what are most comfortable and how to deal with potential hotspots and blisters. A few years back I did a 38-mile inline skating race, and I wound up having to tape my feet with athletic tape to prevent blisters before every training session.

Like with cycling events, you'll want to make sure everything the day of the walk has been tested -- e.g., you don't want to wear new shoes or try new foods at the last minute.

Good luck!