There are three basic running 'thresholds':
(1) The Anaerobic Threshold is when we use glycogen as the main fuel source and lactic acid accumulates in the blood stream faster than it can be cleared away. We can only keep up this level of intensity for a few minutes. (2) The Lactate Threshold is when lactic acid is just beginning to accumulate. Although sports scientists measure this differently from the anaerobic threshold, for the rest of us and for training purposes it is pretty much the same. Both are when we reach 70 - 80% maximum perceived exertion and 80 - 90% maximum heart rate.
Training at these thresholds means we adapt to run at higher intensity more efficiently, and for longer before fatigue hits. Threshold training pace is harder than our normal easy run, but slower than 5k or 10k race pace. You should be able to maintain it for 20 or 30 minutes. It should feel hard work but not unbearable! Once you feel your breathing and exertion is at 5k race level, ease off a little. You can add increasing distances at this pace into the middle of a weekly training run, start at say 20 minutes and increase to 40 minutes.
(3) The Aerobic Threshold is the highest intensity we can run relying on the metabolism of body fats for energy, without lactic acid building up. Running below the aerobic threshold feels comfortable and we feel we can keep going at this pace for a few hours. Aerobic threshold is around 70% of our maximum heart rate. The higher our aerobic threshold, the faster and longer we can run comfortably.
One of the mistakes a lot of us make when training for long distances is to run long runs too hard, at 'medium' effort. This means we're not actually training to increase our aerobic threshold, and we may be doing more harm by training too intensively so that our bodies don't recover well. Research suggests a long run should be at around 65% of 5k race pace. E.g. if your time for a 5k is 20 mins, your pace is 4 mins/km. Running at 65% of this means 35% slower or about 5:24 min/km pace.
Speed work and interval runs raise our lactate / anaerobic threshold so we can tolerate longer periods of intense running. Easy long runs promote aerobic fitness and endurance. If you find yourself doing too many medium effort runs, these are the so-called junk miles. They don't provide the good effects of intervals but cause more fatigue than easy runs. Avoid them!