Most of the walkers and slower runners I've raced with have followed that rule. Maybe 95% of them understand and they don't want to get in someone's way, where I know when I'm trying to put on some speed, I don't want to "pressure" or bother people who are going slower than me. This morning before the race I noticed they did make a couple of announcements asking people to stay to the right, and asking walkers not to walk more than two abreast. Happily, it wasn't as congested, because by the time we reach the choke points on these longer races, the crowds have gotten strung out.