You don't need extra carbs for these run distances. Much of what your heart rate is during a run will be effected more by the temperature, your body's ability to cool itself, if you've just had a cup of coffee or how rested and stressed you are, etc. I keep my workouts to their intended purpose. When I do my daily easy run, it's just that. I record heart rate to grade the effort, but the run is done by perceived effort. I run what feels easy that morning. I do interval workouts to work conditioning at that effort. Whether it's with kettle bells, run sprints or bike intervals. By working the anerobic system for short sprint intervals and then conditioning with a daily easy effort hour run the heart rate improves and overall fitness is better than if you just did easy runs or just did HIIT. All this is my opinion and what my training has developed into.