Anyone who works out or does cardio workouts of any kind really should invest in a Heart Rate Monitor Watch. They are great for tracking your heart rate as well as other important information. A lot of them use a GPS to track the distance you walked/ran as well as your pace and time. Data from each run is then stored into the unit so you can go back and review vital details. Some even do calories burned as well which I know is a great motivator. Sometimes you don't even have to run a long distance or walk a lot to burn the calories you want to. Sometimes you just need to burn a couple hundred calories a day and walking 2 miles at a brisk pace can easily get you there. If you can achieve the speed of walking a mile under 13 minutes then you will burn even more calories.
A heart rate monitor is very vital for certain people. You will know what heart rate your limit is at and you will know to never go over that rate again. If you are approaching it you know to slow down or even take a break. Losing a few extra calories or getting in that extra half mile is not worth a heart attack or stroke! So get your watch and start tracking your cardio!