If you have access to a treadmill, focus on the pace guidelines, working at your own intensity level. If you run outdoors and do not have access to any tools to measure your pace, then use the intensity guidelines (rate of perceived exertion) as a guide for how fast or slow to run.
Beginner Interval Running Workout
Intermediate Interval Running Workout
Advanced Interval Running Workout
An Explanation of Using the RPE Method to Measure Intensity
Rate of Perceived Exertion (RPE) may be the most versatile method to measure exercise intensity for all age groups. Using this method is simple, because all you have to do is estimate how hard you feel like you’re exerting yourself during exercise. RPE is a good measure of intensity because it is individualized—it’s based on your current fitness level and overall perception of exercise. The scale ranges from one to 10, allowing you to rate how you feel physically and mentally at a given intensity level.
An RPE between five and seven is recommended for most adults. This means that at the height of your workout, you should feel you are working "somewhat hard" to "hard."