I've only tried a half marathon on my own. It took me quite a lot of time getting to that point with conventional training. Latter I dropped running because of knee problems and didn't run for years.
Then I read about the tabata protocol (http://www.pponline.co.uk/encyc/0145.htm
) and it's ability to also train for endurance, albeit you never do any endurance work. I was very skeptical, but gave it a chance: I did a tabata run approx 5 days a week for 3 weeks, rested a few days and tried to test my endurance in a longdistance run. I EASILY ran 15-17 K ... that is after a pause of 4 years of not running, alot of biking and strength training though.
My best guess is that the WOD, combined with tabata running and rowing and a weekly long run is the recipe.