One of the keys to achieving your best marathon time is to keep an even pace throughout the race. Go out too fast and you’ll hit the wall, go out too slow and you won’t be able to make up the time later (but you may feel more rested at the end). Determine the pace that you believe you can honestly achieve and stick to it.
Use these wristbands to guide you through those miles (or kilometers) when your mind can’t do the math anymore (or you want to save your concentration for your race). Of course you may need to modify these numbers to take account of the local terrain – but these pace bands will help you along.