Best bet is to spend the next 15 weeks experimenting with diet and meal timings to see if you can solve the problem.
What I do now, after a couple of close calls,
From wednesday in race week
Cut out any high fibre foods
Cut out fruit, especially dried fruit inc. muesli.
Cut out spicy food
Cut out high fat food
Cut down on milk / lactose
Means a very boring and bland diet for a few days but no toilet worries.
May also be worth experimenting with the different variants of Imodium and also the supermarket own brands as these seem to have a less powerful
effect so may help.