Just grab a basic Polar Heart Monitor. They're great. I think mine is a T13 or something - very basic - it was thrown in when we bought our treadmill. It tracks how long you've exercised for, calculates the average heart rate, and also has the ranges so you can make sure you're between two values ie if you want to make sure your heart rate is between 100 and 150 beats per minute. I love it. Have a look on Ebay. I have thought about getting one to calculate calories and distance as well but can't justify it at the moment.

HTH

MG