Listen to yourself

And not necessarily to technology. Yes, like Jeff Gaudette from competitor.com, I too agree that GPS watches, heart rate monitors, etc. can be very useful training aids. I analyze data from "workouts I care about" over my peanutbutter sandwich at lunch... how consistent was my pacing? how much did I slow down on that hill? how long did it take my pace to recover? did I improve from the last time I ran that route? And I don't even have a heart rate monitor! However, I want to point out something important, I do this AFTER the workout. Yes, I might glance down at my watch every once in a while during the workout, but I try and treat it as a "normal" watch and run mostly be feel. Why? First off, technology isn't perfect. Second, even if it is recording correctly, it IS normal for your pace and heart rate to change over time. Aside from terrain, navigating turns may cause your pace to slow slightly, and a tail wind cause you to speed up without any increase in effort. With respect to heart rate, there are many factors that affect it (sleep, weather, hydration, etc.), and it is not well-correlated with other training indicators (lactate threshold). Also, it is normal for your heart rate to drift over the course of a run, even if your pace and effort level doesn't increase.