There's so much variety with intervals - if you have access to a track or use a garmin, something as simple as running 200m or 400m with a jog recovery constitutes interval training.
For example 12 x 400m with 200m jog recoveries, but the aim is to do the last 400m the same speed as the first! Using your HR as well, you would expect your working heart rate to be up at about 95%, and on your recovery making sure it drops to at least 70%.
Either that or timed runs, e.g. 3.5 minutes at effort (95% WHR) and 1.5 minutes jog recovery (<70% WHR) x 6 times.
You should find that intervals really improve your running performance
