In training, you learn to account for everything: how and what to eat and drink, how to incorporate strength work, yoga, stretching, running etc. into long-distance bike training. Training is where you get strong, find your limits, get confidence, get your mind right work out your system. All of the ride day details get dialed in practice: which bike (mtb or gravel), pack/no pack, how many water bottles, what size bottles, Strava/no Strava, Strava on phone/bike computer, shorts, bibs, jacket, which helmet, shoes, snacks, sunglasses, contacts/glasses, it seems endless, but you get to decide which item makes you feel better and ride better. The only pain you should feel is from your physical effort, not from your equipment.
You need to have been through all of this in practice, that's why training is so important. The good news is that all of these things are about your comfort, dry and comfy makes for a happier rider, so start there. Waterproof boots and gloves can be the difference between victory and misery.