The Marathon Predictor Tool
This tool is based on the work of Giovanni Tanda and Beat Knechtle, published in the Open Access Sports Journal, under the title Marathon performance in relation to body fat percentage and training indices in recreational male runners.
The study was performed on men, so if you are a lady, I can't promise that this will be accurate! Ranges are also given for the result to still be valid - if you fall out of the ranges, the accuracy will also fall off, but might give you a ball park figure.
Your predicted marathon time is: