A group process is like a marathon.
At the beginning, there's anticipation, excitement, energy, and a little fear. In the middle, there's exhaustion, fatigue, and doubt. Some may wonder how they'll ever make it to the end. At the finish, there's exhilaration and a sense of accomplishment.
People enter into a group process at different places. At a facilitated meeting, some participants may have already been dealing with the subject for weeks, while for others, it's the first time they're hearing about it. Those who have been a part of the process for sometime may have forgotten the fear and pain from earlier. The principle of the marathon reminds participants that others may need time to process what they're now a part of.
I first heard the principle of the marathon from Nancy Reuscher.