I have run 15 marathons and finding the perfect pace has always been a problem. I believe that I have a formula for marathon pacing that works. I will explain how to calculate your pace using a heart rate monitor.
The major problem running marathons for me has always been starting out either too fast or too slow. When I start out too fast, by mile 18 I am in serious trouble. With eight miles to go and my body in bad shape things get real ugly. When I start too slow by mile thirteen my legs will usually not respond to the faster paced. I believe this happens because as the legs become tired your stride length and turnover will not increase to a faster pace.
To determine your marathon pace run a half marathon at your best effort during the first week of your marathon training program. Wear a heart rate monitor during the half marathon and record your average heart rate of the entire race. By subtracting 5 beats from this number you will know your marathon pace heart rate.
Example: average heart rate of 150bpm – 5bpm = marathon pace heart rate of 145bpm
Training at this heart rate once a week during your marathon buildup you will learn to run at marathon pace and the correct effort. Start your marathon pace runs at 5 miles and build up to 15 or 20 miles in length.
Your long runs can be used as marathon pace runs as well. Every other week running a long run and adding marathon pace to the second half will improve you fitness tremendously. Once a month use one of your long runs exclusively as a marathon pace run. After a longer marathon pace run over 15 miles use the next week to recover.
At the midpoint of your marathon training run another half marathon and record your average heart rate. Doing this will allow you to monitor any changes in your average heart rate. If your average heart rate increases or stays the same you will know that your training is going well. If your average heart rate goes down, you will have an indication of over training.
As your marathon race date approaches run a half marathon at your best effort three weeks before race day. Use a heart rate monitor during the race to record the average heart rate of the entire race. After the race is over save this number for future reference. Take the average heart rate of your half marathon and subtract five beats from it. This number will be the maximum heart rate for running you're upcoming marathon.