Monitoring your heart rate during training sessions has two main uses: observation and control. Observation is the more straightforward of the two, and you can do it whether your heart rate monitor (HRM) is an entry-level unit, displaying just your pulse, or the Rolls-Royce type that you can use to download data onto a PC. However, the more advanced your HRM, the easier it is to observe your heart rate.
If your HRM doesn't record and store your heart rates, then try to take a look every few minutes as you're running, to get a general idea of your rate throughout the session. You will probably find that, even if you maintain the same pace throughout, your heart rate will rise by 5-10 beats over a period of 30 minutes. In physiologists' jargon this is known as cardiovascular drift, an upwards trend which is caused by two factors. First, as you run your muscles are performing a lot of metabolic work, and this increases your heart rate. Second, you are likely to become a little dehydrated, which leads to a small drop in blood volume. This means that in order to supply the same amount of blood to the muscles, the heart has to pump a bit faster. In short, don't be surprised if your heart rate creeps up a little during a run, even if you maintain level pace.
Your running programme should include a variety of training paces, from the long, steady run to threshold sessions or track reps. Use your HRM to get an idea of how your heart rate varies between each of these sessions. If it doesn't vary, then you're falling into the classic trap of running your steady mileage too fast, a common problem in the motivated runner and a passport to overtraining and its associated doom and gloom.
On a day when you're a little tired, you will find that your heart rate is slightly higher than normal when running at your usual pace. This may be accompanied by an increase in resting heart rates. Both these increases are probably caused by a couple of days' hard training, or perhaps by a race or track session the day before. You should regard this as a gentle warning to take it easy; keep to your normal heart rate and run a little slower, as this will help you to recover before your next hard session.
If your overtraining is more chronic and you have been suffering tiredness for a few weeks, you may find that you struggle to get your heart rate up to the normal levels in your training runs. This is a stronger warning that you've been overdoing it, and that you need proper rest. This means taking a few days off, or at least restricting yourself to easy jogging.
By carefully noting your training heart rates, you can get a good idea of what rates to expect, and what is too high and too low. This is easier if your heart rate monitor allows you to view your data after the run, as you can then analyse strategic points on your training route.
It's worth pointing out at this stage, in case you hadn't already noticed, that your heart rate will vary according to where you run. For instance, running up a hill clearly involves greater mechanical work than running on the flat, and thus causes greater physiological stress and the associated higher heart rates.
Not so obvious are the subtle changes caused by different surfaces. Run seven-minute miles on nice smooth tarmac, and then do the same on grass or, worse still, long, wet grass or sand, and you may think you've upped your pace by half a minute a mile the heart rate will be that much higher.
So when logging your training heart rates, take care to note the surface as well as how hilly your run was. Otherwise you might diagnose yourself as exhausted simply because you ran on wet grass instead of your normal road surface.