Way too many variable here to respond very well. BUT, if you can run comfortably for 18 miles and do it three or four times before a marathon, and IF you have the absolute willpower to run at only about 65% of your 10K race pace, you should be able to finish a marathon without two much trouble.

Remember, DON'T get cocky. For most people, the race doesn't really begin until mile 20.

Tyson