Originally Posted by TejanoTrackie

The proper way to do endurance interval training, is to first warm up at a slow pace, and then gradually build up the effort until you max out. Suddenly starting fast guarantees that you will build up lactic acid quickly and not be able to go for as long, before taking a break. It would also be helpful if you wore a heart monitor, so that you can establish your fitness zones. You want to set your workouts based on time, not distance. As your fitness improves, you will be able to go faster longer, while maintaining the same target heart rate.