I'm currently training to run in a specific HR zone (135 +/- 2 at the moment), in order to build up aerobic endurance. I'm observing that the pace with a given HR is constantly increasing during the workout - if during first 5-10 minutes of the workout I'm able to keep pace around 7:50, closer to the end it falls to around 9:30. I have tried to split running into several 15-20 minute blocks with 5 minutes of walking in between, the performance is still the same.
Are there any techniques or workout plans, apart of just keeping running in a zone for extended time, that may help to get to stable performance while running in a heart rate zone?
Thanks in advance for your answers!