Plenty of whatever chamois lubricating cream takes your fancy; slap it on and take a bit extra. You’ll get chafed from a combination of salt from dried out sweat and grit flicked up from the road if it’s a rainy day, so plaster it on.
Drink little and often, eat little and often, don’t stop for too long and don’t set off too fast.
Think about the weather, dress appropriately, aim to be warm rather than dry, use socks over your shoes to keep the grit out; wearing overshoes in anything other than torrential rain just makes for sweaty feet.
Take a spare pair of gloves, a spare undershirt if you’ve got support, and give me a shout if you need any wet weather gear.
…and think carefully about wee stops, lot easier for chaps…