To go fast you have to go slow which sounds wrong but it is true. Bobby McGee and Mark Allen emphasize this for your training of running and triathlons. I do 1 speed workout for swim, bike and run each week but all other workouts are in Heart Rate zone 1 and 2.
Case in point, 2 weeks ago I ran the Yankee Homecoming 10 mile road race which I ran 9 minutes faster than last year. In fact I have not run that fast in the last 8 years for that race. It was perfect summer evening for the race with temperatures in the 70s and cloudy. Throughout the race I was patient and present in the moment.
Maybe KT is right, all this training for the Half Ironman has given me this incredible endurance. In the swim, I focus on my form and force of the pull. In biking, focus on strength on the short ride with hills and endurance on the ride. In the run, hill repeats or tempo run, long runs and easy transition runs.
Each of your endurance workout you should be focus on what you are doing now, not what is next or what happened. When you get present, the miles flow off faster. So go slow you will be surprise that you will be faster.