Wow, you just finished a half-marathon. Congrats, I'll give you all the credit in the world for doing that as I get winded walking to the fridge. You deserve a pat on the back for being able to run for a kind of long ways.
But don't do this.
Don't finish the half-marathon, shrug your shoulders, make a disapproving grunt and then turn around and continue to run to show off that you're in such good shape you can just keep going.
If you are in that good of shape, just do a full marathon. Nobody cares.
The "extra laps half-marathon" guy is a scourge to those that showed up just to run the 5K fun run, or barely get through a half-marathon. They don't like these show off guys, in fact, they secretly hope that these show off guys roll an ankle.