20 minute lactate threshold (LT) intervals are the base of interval training. LT is where you are almost, but not quite panting. Harder efforts will have you panting by the end of the interval, but not at the start. It's important to hold the same effort for the whole duration of the interval, and to stop doing them when you can't hold the same effort you were making on the 1st and 2nd interval. So it takes a bit of practice to figure out what those efforts feel like, with or without a heart rate monitor.