Run Long, but Not A Lot
It may be intuitive to believe that running a lot of miles is an effective means to improve one’s chances of running a strong Ironman marathon, but in fact it is not. While you have to be fit enough to run a strong marathon, you should actually do the minimum amount of running to ensure that you are capable of running a strong marathon and devote any remaining available weekly training hours to building the bike strength that will enable you to actually realize your running potential.
I recommend that even serious competitive Ironman triathletes perform only three independent runs per week. The most important of these is the weekend long run. Complete at least four runs of 18 miles or more, and feel free to go as long as 26.2 miles in training to cement a solid reserve of running endurance. Put as much as you want into your long runs, within reason, but resist the temptation to do any more running during the rest of the week than is required to support your progress in these long runs, as it will only increase your risk of injury and burnout and take away from your cycling.