Runner's World has lots of neat stuff; LOTS! One particular item is a training calculator that calculates times for different distances based upon a time of any given distance.     For example, I put in 53:18 for a distance of 10K and it calculated the following:

1 Mile	7:42
5 Kilometers (5K) (3.1 Miles)	25:35
5 Miles	42:22
10 Miles	1:28:19
Half Marathon (13.1 Miles)	1:57:35
Marathon (26.2 Miles)	4:05:10

The really wild thing is that when I ran my 5K race on Sunday, it was within 1 second of the calculator.     Based upon the calculator, if I continue to train and train smart and learn to be more consistent, a sub 4 hour marathon looks achievable.     Rock on!