I'm another one who's had two colonoscopies. Try to schedule it for the morning so that most of the fasting time will be while you're asleep. The prep is not a lot of fun, but it's only about half a day and it's very much not as bad as gastroenteritis or anything else that'll give you a good bout of diarrhoea.

And if you aren’t a fan of being sedated (I’ve never been, and I’d prefer to keep it that way), you can opt to do it without sedation. Had a colonoscopy and an upper endoscopy the same day, no sedation. A little uncomfy, but not painful. If you can push yourself through hard weight training sets, or run a 50k, or anything else that entails a bit of discomfort, a colonoscopy is no biggy.