Honestly, I’d just decide on the day, but a couple of pieces of advice from someone that has been through this before:
1. Stay hydrated. This is hugely important! For every litre you flush out of the bilges, you need to be drinking a litre to replace it. When things get heavy, you’ll find it hard, but trust me – it’s a lot better than how ill you’ll feel from being dehydrated!
2. If losing a lot of fluid, make sure that your replenishment fluid includes salts – you can get special solutions from the supermarket to help you with this.
3. If you decide to ride, carry Imodium Instants and don’t even be scared of taking a couple 30 mins before the start if you’re worried. They should easily last you the length of the ride and will give you peace of mind without causing any harm
4. Carry a small pack of tissues. If the worst comes to the worst, you can always jump a wall 🙂
Otherwise, just steer clear of other riders as these things are typically passed on by touch and if you’re feeling really poorly on the day, just skip it – there will always be another one.
Hope you manage to make the line!