Here are some tools to help you out with those pesky calculations

Data Geek Zone

Splits based on marathon time
Enter finish time in H:MM format

Conversion between pace and speed
Press + or - keys to adjust the increments

Marathon Predictor
Enter time for 5K, 10k or half marathon

Weight Convertor
Enter kg value

Run paces, enter target marathon time
Approx paces, to be used as a guide

Adjusting your time for various conditions
Set off at an easier pace

Nutrition Requirements
Approximations, test in training

Marathon Splits
Various ways to get to the finish