Are you training with a heart rate monitor?

Heart rate monitors are one of the most important things to have on a run. I never leave home with out it. You monitor will help you get to know your body better. It will tell you if you're in your aerobic zone or your anaerobic zone. Personally, I can get my heart rate up really high in a spin class but not as high when I'm running. My husband is always amazed when he downloads my workouts to the computer because when I'm racing it's in the 180's the entire race (5km). On a training run, I use it as a guide for my level of effort. If it's in the 150's I'm having an easy day, 170's means it's a hard day, and 180's+ means on the verge of puking. Everybody's heart rate will be different. You can calculate your zone by subtracting your age from 220 and then multiplying that number by percentages - 60%-90%. This is an old formula and I don't think it's that accurate, only you can tell what your effort level is, but this is a good place to start.

I wrote more about heart rate monitors here.