I'm looking for advice on how to do interval training. I've been cycling recreationally for many years but only using it for training for the past few years. I want to increase my speed (say, on a 10 or 15 mile moderately hilly time trial) and I've been told that the best way to do that is interval training. The thing is, everywhere I look they suggest doing different things: 30 second intervals all the way to 15 minutes, at different intensities, with different recovery times. What kind of intervals should I do? I have a heart rate monitor but no power meter. Thanks!