Heart Rate training is now becoming more common. The gold standard for finding your maximum heart rate is a treadmill stress test in a lab, but you can simulate one on your own with a heart-rate monitor.
At a track, do a warm-up mile or two, followed by a mile at tempo (comfortably hard) pace, then gradually increase your speed over 400 metres before running a final 400m all out. The highest number on your monitor will be close to your maximum heart rate. Or race a 5K at your fastest pace (held consistently throughout), running the last one or two minutes as fast as you can. Your heart rate at the end should be close to your MHR.
OR you can use the calucation of 220 minus age etc etc. However, there are many variables you can’t account for using a formular, such as your fitness level, resting heart rate, health, weather (heat and humidity will make running harder.)
Here is a great article from Runner’s World on the subject