There were some requests to get some more info on my interval specifics. I use a treadmill because it is so much easier to set speed and time than running out in the real world constantly glancing at your GPS watch.

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This heart rate data includes my warm up (3 min walk then run 1 min at 5, 6, & 7 mph with 1 min walk between) and cool down (3 min walk) and 3 miles of actual interval running (3 min at 7 mph and 2 min walking both at 0.5% incline).