+1 to rplong, your long weekend runs should be around 20+ miles. a 3 hour marathon is a pretty big goal, 7:15 is really fast if you aren't even up to the distance yet but durning races you can run alot faster with the motivation of others.
Personally, I have trouble during thoes last 5-7 miles, I just get sick of running and easily distracted(or focused on the pain)