If you have found that you wouldnt find a 30 minute jog/walk as challenging its time to step it up. Interval training involves cycling exercise pace/intensity to create a high/low wave effect on your heart beat. To fully understand how it works you must understand the RPE scale (Rate of Percieved Exertion). The RPE scale is an imaginary scale where you describe exercise hardness from 1 -10. Imagine 1 as being very easy (light walk), and 10 being the sprint for your life. You would not able to maintain a level of around 8 or above for a long period of time.
This describes example 2 in the graph. Start off at least 5 minutes normal pace to warm up, about 5/6 RPE ( This is aerobic exercise, as your muscles have time to take in oxygen and exchange for carbon dioxide). Then for cycles of 1 minute out of every 4 minutes increase the pace rapidly to around 9 (you should only be able to maintain this pace around 1 minute!). In this phaze your body can no longer supply adequate oxygen to your muscles to complete full contractions (your breathing is at full lung capacity, you cant take more air in!), so you go “anaerobic”, and you start to feel the burn as lactic acid is a by-product of this energy process.
After 1 min (you should be very tired) of 9 RPE, drop back to your normal pace. This is called your “active recovery” phaze, as your body can supply oxygen to your muscles, flushing out the lactic acid. Repeating this process will create an aerobicwave effect, that is good for training the heart to work at high and low contraction, which is great for burning calories and improving cardio-vascular health and lung capacity.