The obvious part about training is to get your body in shape to be able to handle three straight days of walking almost 20 miles each day. Everyone understands that. However, there are some people who feel "I run on a regular basis", "I work out every day", "I'm in great shape" and they think they can get away with little or no training and still do OK on the walk. The truth is that physically they may do fine, but there's so much of the walk that has nothing to do with your physical ability.
Let's stick to the physical for a minute though. First, running is not walking. Working out, is not walking. Walking 5 miles every day is NOT the same as walking 10 miles. Walking 10 miles is not like walking 18 or 20 All of these other activities can contribute to it being easier to do the 20 miles, but if you don't train for walking, and you don't train for walking long distances, you really won't learn what you need to know to make it through the 3-Days. I remember, the first year I trained. Getting to the point where doing a 10 mile walk seemed kind of easy -- I had it in my head that all I needed to do was do that twice to get through each day! I then was astounded to find that adding two miles to that, made it exponentially harder. Later, 18 miles seemed doable. But adding a 15 mile walk on the day after I did an 18 mile walk was tough. My point is, that the more you are able to follow the rough pattern of the training program that Komen has laid out (either the 24 week or 16 week program), the better off you will be on event. I've never followed it 100%. Life gets in the way of training. But each year I've done a better job of training harder and sticking to the schedule better, and each year, the actual even has been easier for me.
I recommend starting informal training as soon as you can in the year. I was in pretty bad shape when I first started doing these walks and so I would start with one mile. I gradually increased that until doing 3 miles was almost as easy as the one mile had initially been. I then got to where 5 miles felt like a piece of cake. I tend to sustain that -- trying to do 5 mile walks at least 3-4 times a week throughout the year, until the formal training program starts.
But lets say you are already a walker. Let's say that part of it is easy. Here's the other, and in some ways more important reason that you should spend time training. Training is the only way you can figure out what gear will work for you. Do you have the right shoes, the right socks, the right clothing, the right bag to carry your water and sports drink? What other items are crucial for you to be able to carry with you to get through a long walk? Training is the only way you will know if your feet are prone to blisters, if changing gear can change that, and if not, how you cope with, prevent and care for blisters so you can keep going. Training is the only way to learn how you need to hydrate and how to get your body used to going about 3 miles in between bathroom breaks. Training also lets you learn what keeps you going when you don't feel like going anymore. It's a difficult rule that they do not allow you to wear headphones on the walk, but music makes all the difference for me when I feel like I want to give up, so I carry speakers that I can clip to my bag and I save the music for those times when I absolutely need it on the event. (I do admit that I train wearing headphones quite often -- but I usually only put one in so that I can still hear what is going on around me and keep myself safe.)
I hope that this gives some insight into why we want to devote a significant amount of time to training. The more you do, the more you are able to work out what works for you and what doesn't. What your body is capable of and what your mind is capable of.
May your new year be filled with walking and may you never get a blister.