Have a good sleep schedule that allows your body to recover.
Just basically take very good care of your body.
Eat well as the bro's advise and also some of the extra vitamins advised.
Plenty of fluids and very little booze.
5 is a good goal.
Im planning a punting trip also.
I am doing extra cardio excersise which I know will help.
Listen to your body while on the trip,if ya get tired and 4 is the max then rest it and hit it again the next day.
Bearcat