Do Frequent Transition Runs
In Ironman training, a mile run immediately after riding is probably worth five miles run on fresh legs. Running off the bike in training prepares you specifically to run off the bike in an Ironman. I believe that doing short runs frequently off the bike in training is more beneficial than doing occasional longer runs off the bike, because it’s really the transition from cycling to running that you are trying to train. If you can start running strong off the bike, chances are you will continue running strong. And the converse is also true. Unlike in regular marathons, Ironman marathons usually don’t turn ugly at 20 miles. They start ugly.
After building your base, add one easy mile of running after one ride in the week. Then add an easy mile of running after a second ride the next week and continue in this manner until you are running an easy mile after each ride. Next, add a second mile of easy running after one ride, and the following week add one easy mile to another post-ride transition run. Finally, over the ensuing weeks, gradually increase the pace of those two two-mile transition runs until you’re doing them at roughly your lactate threshold pace.
A second advantage of this approach—the first advantage being its effectiveness—is that it is not terribly stressful. If you ride four times a week, you are looking at a maximum of six miles of transition running, with four of those miles at threshold intensity. Those few miles will do you far more good than they would if incorporated into your training in any other way.