See a good physio or osteopath. Back problems can be caused by minor aggravation of long periods of time, an unbalanced pedal action, lack of core strength, etc, etc, etc. So go get sussed out by the healing hands.

Days. As in you should be loading up with water, carbs, and an appropriate amount of salts in the days leading up to an event especially if its a long distance thang.

On the day I prefer to just do things normally. ie. eat a decent breakfast, keep topped up with fluids (water, juice, sport drink, whatever turns you on at the time), and snack properly (eg. fruit, sambos, regular stuff). Try to save the sugary stuff for later in the race, otherwise your palate will be totally turned off the taste or worse you may hit a sugar low (is that an insulin overload?) which can really knock you about. I've done that twice and its not pretty.