Protein is not primarily used as a fuel. It is only used as a fuel when your stores of carbs are depleted. At this point the protein is taken from the muscles (yep, not good really) and yelds the same amount of calories as carbohydrate.

Because of this an endurance athlete actually has a higher requirement than a bodybuilder. (but bear in mind that most bodybuilders take far more protein than they need to!)

The flip side to this is that you dont want to take too much protein.(Quote from Nutrition for serious athletes by Dan Benardot) "Since high protein levels result in either the excess protein being stored as fat or, if there is a general energy intake deficiency, being burned as fuel, the nitrogenous waste products must be removed. This excess urea excretion causes a greater urinary volume to be produced, increasing the chance for dehydration."

Dan suggests that 1.5grams of protein per kilo for endurance athletes, which compares to the recommended 0.8 for non-athletes.
So if you weigh 75 kilos, then you should consume 112.5g of protein each day.
Incidentally, carbohydrate is suggested at 9-10g per kilo per day, giving the same person around 700 grams of carbohydrate per day.

Hope this helps.