Most people going into a marathon are very well informed when it comes to training regimes and increasing their fitness and endurance. However their strength training falls by the way side...
so, I will keep this short - but do your research at the very least make sure your legs are strong, your knees are strong, and your gluts (bums!) strong.
... and don't forget the hamstrings and calves! they're pretty important too!
if you want more details. feel free to ask away!