When doing this type of analysis there are three big factors involved: whether or not they are equally as well prepared for each race (their half marathon and marathon); if the course and weather conditions are the same or similar; and the the natural predispositions of the runner (physical make-up, personaility, etc).
So we would have to make some assumptions since I don't know all the courses, weather conditions and preparedness for all of the races in question.
But if we look at a 69:00 half marathon, under similar conditions and preparedness we get the following for the marathon, depending on predisposition:
Neutral Predisposition: 2:24:10
Moderate Long Distance Predisposition: 2:23:30
Moderate Short Distance Predisposition: 2:24:50
For a 68:15 half marathon
Neutral Predisposition: 2:22:40
Moderate Long Distance Predisposition: 2:22:00
Moderate Short Distance Predisposition: 2:23:20
I already have the math worked out and charts with these type times on them on my website:
www.elitemarathoning.com
on the Training Pace & Times page.
Caution: make sure you have taken into account all the variabes (preparedness, course, weather, predispositions) before trying to draw any conclusions. Also remember that not all race tactics will result in a maximumal time performance. But having said all of that, these charts serve as a very good guide to show you the possibilities given similar conditions and preparedness