Since most of us tend to speed up our walk as thoughts carry our attention elsewhere, the deliberate slowing of the pace every 10 minutes helps to bring our attention back and restore a slow-ish pace for the meditation. Even if you tend to walk slowly anyhow, the deliberate slowing of the pace can add a new element to the meditation.
There are no right or wrong ways to do a meditation. The instructions are guidelines; adapt them to who you are and to your needs at that particular time. Be curious about the process itself.
Remember most meditations become richer the more you practice them. They reveal more of themselves. It can take practice to remember to do a meditation when you need to, and it can take practice to go through the steps. But that's why it is called practice, and for most of us, we practice for the rest of our lives.