Something to think about is how efficiently can you pedal? Economy of motion is one thing that can either lead you to the front of the pack or leave you hanging off the back end.
Most cyclists these days have heart monitors if they are at all serious about improving their performance on the bicycle. Generally, higher heart rates indicate greater stress on the body. So it would seem that if one rules out other issues that can cause high heart rates such as illness or other stresses in your life and increase physical exertion, then one could use the heart rate monitor as an indication of how hard you are working. Just be aware that the heart rate changes over time and is not an instant indication of how hard you are currently working. You will need to make a constant level of effort for several minutes to get a more accurate indication of how your heart rate is affected.
Inefficient pedal strokes mean applying more physical stress to the pedals but not all of that stress goes into moving the bicycle forward. One quick way to determine how efficient your pedal strokes are is to try pedaling as fast as you can until you start bouncing in the saddle. Using this cadence speed as a future reference, one can check to see how much better your efficiency is. Try this exercise again but instead of using the ball of your foot to apply force to the pedals, try moving your foot back on the pedal until your toes are over the pedal axle and see how that can smooth out your pedal stroke and result in a higher cadence without bouncing. Some cycling authors keep saying there is no point to using your ankles while pedaling and that it is a waste of energy. However, if it does improve your cadence, how can it be a pointless exercise?
There are other things that also play a big role in how high your heart rate goes. Things like breathing exact a high energy cost when done poorly. If you select a speed to cycle on your indoor trainer (we want a relatively constant amount of resistance for this test) and try changing your breathing, you will quickly begin to realize how even breathing can reduce cycling efficiency.
Test this concept yourself. Here's how you do it.
Try cycling at a constant speed while changing your breathing. Try long shallow breaths at an easy pace such as Heart Rate Zone 2. Then try short, quick inhales and long exhales. Try changing the length of your inhale and exhale. When you are able to cycle at the lowest heart rate you have maxed out your breathing efficiency at that speed.
Just as higher intensity cycling changes your breathing, you may also find that you have to change the way you breath as you approach your anerobic threshold. I find that a strong, quick inhale for a count of one, and a strong exhale using stomach muscles to a count of two does wonders for boosting one's aerobic capacity. But wait... there's more.
One's position and fit on a bicycle also affects your heart rate. Try leaning on the handle bars with your forearms and note the change in heart rate. For me, this can result in roughly 5 beats per minute by lowering my heart's height above my core so the heart doesn't have to pump so much blood uphill. Taking into consideration that the lower one's heart rate is for endurance events in particular, it makes sense to assume a position on the bicycle that causes less physical stress (and pain) so your endurance can be maximized. You might also find that sitting perfectly upright lowers your heart rate since your core muscles are not being stressed as much to hold you upright in the saddle.
Using a heart rate monitor, you can work at improving your efficiency in more areas than at first seems obvious. Maximizing your speed and minimizing your heart rate can often give you the added performance boost you need to do well on long rides. Give it a try! You only have to take a few minutes to decide how you will test a certain aspect of your performance on a bicycle and a heart rate monitor is an inexpensive way to optimize your overall efficiency.