As noticed before, I'm not a certified coach nor a personal trainer so take this advice at your own risk.
It is a known fact for people who train a lot, that is easier for a person that has never ran a marathon to become fit for that marathon than for somebody that is fit to improve his or her time on the same marathon. And there is a simple explanation for that. We are used to keep a certain routine, and since most of us train on our own, we follow things that have worked for us before. We have a training plan and we tend to stick to it. That plan gives us results, it works and works and works. Except when you reach a plateau in your condition and you stop improving. Training is and act of faith. We don't know the results until we prove ourselves, but in order to test that we need to train first. So, we believe in our training plan and because of that we stick to something known. And that's when we get stuck on the same things and we do not improve our times.
There are signs that tell us when it is time to introduce variations in our training. One of them is when it gets hard to reach a very high heart rate. You might be running fast and long, and yet your heart rate stays under control. You might be spinning like a madman and yet your heart rate stays under control, unless you start using ridiculous loads, which most likely are going to lead to injuries or overload. And then the season will be over before you know it. So what to do? That is when qualitative training comes to rescue. We are usually so focused on heart rate and interval duration that we forget to listen to our body. Not everybody was born to be a super athlete. Most of us are never going to be in the Olympics, so it is unreasonable to think that we will manage that level of training (specially when there is the job thing and the family thing thrown in the mixture). Training is not only following the hard facts, there is a lot of soft facts that will help you improve. Today I want to talk about the "Rate of Perceived Exertion" or RPE. RPE is just a scale that scores the level of training based on how hard the exercise feels on the body, not on your pulse. You can of course keep track of certain thresholds by correlating RPE with your pulse, but the point is that RPE is about how you feel the exercise. The most common RPE scale goes from 1 to 10, where 1 is a very light walk and 10 is something that cannot be hold for very long. Using RPE you can graduate the level of your training. Specially at this time of the year when most people start to feel panic because they realize that no matter what, their heart rate does not go up as it used to be, or because they just keep running and they do not drink all the water in the bottle as they used to (congratulations, you have reached a good level of fitness btw).
With this I want to say, that now it is time to focus on the feeling. After all, when the big day comes, it does not matter if your pulse was above or below your average. It is the time after crossing the finish line that really matters (and of course the fact that you crossed the line by your own means). At this time of the year is when you should start focusing on finding the "right" tempo, the tempo at which you feel that you can keep going the whole distance of your event. You are fit by now, so now the fine tuning starts.
How to do it? Well, start by having more sessions with long intervals and short periods of recovery. My favourite are 12 minutes work and 3 minutes recovery. Try to make sure that during those 12 minutes you reach the level where it is both comfortable to be in yet still challenging. You will probably reach that level without reaching and incredible high pulse and that is okay. Throw in at least one session of really hard intervals. Usually short ones, such as one minute really hard work and one minute relaxation. I have my personal favorite which is to run for one kilometer using poles and as fast as I can uphill, and then having a three minutes rest. The key point here is to find a level where you really feel challenged. It might be that you reach a very high pulse or you might not, however it is the feeling that matters.
You should still keep track of your pulse, but use it as an afterthought and not as the driving force behind your training.
Remember, now it is time to find the tempo that will make you a winner. And that is something that you do by finding something that "feels" right, not something that your heart rate monitor tells you that is right.