I've always been a big fan of using heart rate monitors during exercise. It's a great way to see how fit you are and help adjust your pace when your outdoors. Since I seemed to have misplaced my Polar strap I ended up buying a Garmin Forerunner 301. This thing is amazing - heart rate, distance, elevation, and more. It uses GPS technology to track where you've traveled and has a computer hook up to their software which let's you track and examine your progress.
I've included a screenshot below of our walk today, clearly the fort doesn't exist - very cool product.