I'm not a doc either I'm a nurse that works in pediatrics. Both Pepto and Immodium would be dehydrating during a half/full marathon. We see a big dehydrating effect in the kiddos.
I did ask one of our docs who is a runner and he said he would not take either one before an endurance run for the same reason NSAIDS are not recommended. "Our kidneys are very important"
Of course remember we take care of small fry kiddos and their bodies react different to meds.