Well, if you want to get as accurate a test as possible, you should try to find a more controlled envirnment. Like, a stretch of road where there are no lights and you don't have to stop and go. It doesn't have to be super far, you can use any distance...I like to do at least 3 miles. You really want to go all out at as even a pace as possible for the full distance. Then you can log your time, average speed, average heart rate, and max heart rate. Do that each month and you should see an improvement in the numbers.