Also, don't a lot of early finishers walk back so they can run in again with a friend (stepping out again before the chute)?

I don't interpret what you describe as showboating. Maybe I'm wrong, but I think they're just living in their own heads after the race and aren't thinking about anything but recovery.

Also, how far away is the start from the finish? Do they have any alternative path to get back to their vehicles, as bmccasland pointed out? If the start and finish are that far apart, or a if preferred a path back to the start isn't clearly marked, what else are they to do?