So just recently I ran a marathon without training and that went smoothly. However I didn't pee the entire race and had read this could potentially be a hormonal response where the body shuts down urine production. Now i'm thinking of running a 100 mile which I would train some for and was wondering if that would be safe based off the marathon experience. Thanks!
You should definitely pee during any race even a quarter of that length. The duration of a couple rounds in a bush is going to be negligible in the bigger picture anyway.
One of the main reasons why you shouldn't be holding it in that long, is that your bladder will retain a lot of the bacteria that it is trying to flush, and it can end up giving you urinary tract infections, or even push it back into your kidneys.
Another nasty side effect is urinary retention, which is a tensening up of the muscles used to hold urine in, making you unable to urinate when you need to.
Now, combine this with the obvious fact that you will need to hydrate a lot on such a long run, and an adult bladder can typically only hold 2-3 cups of fluid.
If you have any practical questions about the safety of such a run, I highly recommend a visit to your doctor, as they will know more about your body in particular, and can run tests for the uncertainties.
I wouldn't be so definitive about it... I ran a few marathons at ~4hrs and I don't remember feeling like I needed to pee during all for them... I wouldn't say and made a point of not doing either - just not sure I needed it all that much.
Depends partly how you plan things beforehand I would say... are you already well hydrated to start with, how often do you usually pee, et.