If you haven’t read hack #20, Pee on the bike, check it out.
Again, this really only goes for hot and humid races when you’re already totally wet. At first you would have been wet from the swim, then sweating like crazy, and eventually dousing your body with cool water.
So it’s not like a little pee is going to change much, other than make you feel a whole lot better.
In November 2019 I ran the Standard Chartered Marathon in Singapore and due to poor pee planning on my part, really had to go from the fifth kilometer onward.
There were plenty of Porta Potties along the way (and plenty of dark bushes in this night run) that I could have used. But I was shooting (aiming?) for a personal best and really didn’t want to stop.
I thought about just letting loose, but it wasn’t the same as in a triathlon. The main issue was I wasn’t nearly as wet and didn’t really think it would rinse away well, or be very inconspicuous.
As a result, I held it. I held it for the next 37 kilometers which wasn’t really pleasant. But had it been in a half or full Ironman I’m sure it would have been a totally different story.
Contrast that with many halves and a few fulls I’ve done where during the run, usually at the wet aid stations, I relieve myself, along with pretty much everybody else that doesn’t want to take the time to go to some far-away Porta Potty.