A basic stopwatch is a useful tool and helpful for all marathon runners. If you are not sure of a distance, or if you are traveling, you can use your watch and just run for half and hour, or an hour, depending on your training schedule. And you can get a general sense of how you're feeling by comparing how long it takes you to run the same route under different whether conditions, and when you've had different amounts of rest the night or nights before. You don't need a $100 watch to do that. You can buy a simple stop watch almost anywhere, from a department store to a catalog or a sporting goods store, and even an expensive model should last for several years.